Here you get the patched bitcoin client which does work inside of I2P. User giv has taken the original bitcoin client and modified it to work with I2P destinations. This client does work with I2P only mode and in I2P/clearnet mixed mode, which does work as a bridge between clearnet bitcoin net and I2P bitcoin net.
The source can be found on http://git.repo.i2p.xyz/w/bitcoin-qt-i2p.git and on https://github.com/VirtualDestructor/bitcoin-qt-i2p.
More information on the I2P forum thread or in the bitcointalk forum thread.
The user killyourtv provides debian packages of this I2P bitcoin client under http://killyourtv.i2p.xyz/debian/. Please habe a look and use them, as they are far easier to setup and use.

Version 0.8.6 24.12.2013:




Version 0.8.5 13.09.2013:


bitcoin-qt-i2p-master.2013.09.13.zip Sourcecode from git

Version 0.8.3 11.07.2013:


Version 07.03.2013:


Quick howto:
- enable SAM bridge in I2P on, save and restart I2P
- fetch and install orginal bitcoin client
- rename seed.nodes file (I2Pbtc NEEDS a different filescheme)
- fetch latest btci2p from above link
- unzip btci2p and copy over the original bitcoin client with patched ones from the deflated archive
- start the bitcoin client with the option -onlynet=native_i2p to connect only inside of I2P
- wait until blockchain is synched. You can also copy over a existant blockchain, btci2p is compatible with original blockchain format
- use it as a usual, original bitcoin client!

Copied over information from bitcointalk forum:
Running bitcoin in i2p (both Windows and Linux):
1) after I2P was running you should wait a few minutes while i2p is integrating in the network
2) in your browser open the page If the page is unavailable that mean I2P was not running (maybe some errors were occured). If the page opens you should press "Start" button near "SAM application bridge" item and enable a checkbox in column "Run at Startup?" After that press "Save Client Configuration" button.
3) delete (or rename) file peers.dat, it's located at C:\Documents and Settings\\Application data\Bitcoin (for Win XP) | C:\Users\\Appdata\Roaming\Bitcoin (for Vista and 7) | ~/.bitcoin (for linux). You must to do this because serialization format of network addresses is incompatible between the vanilla client and my patch. You need to do this procedure only once before the first start of the patched client. The patched client will make his own peers.dat with required format. Warning! If you leave this file at his place it cause a deadlock of the patched client.
4) download the patched executable files or compile the executable files by yourself and run it
bitcoin-qt -onlynet=native_i2p
If you want to deal with i2p-net only you should specify option -onlynet=native_i2p as shown above. If you want to deal with both i2p and ip(usual internet) don't specify this option at all, in this case you client will be a gate between i2p-segment of bitcoin-net and the rest net.

If you want to help other bitcoin users in I2P you may send me or publish here your dest-hash or b32-address and I will add them to the sources as seed-node in a next release. But if you want this you should run your wallet with a permanent i2p-address (see section "Starting wallet with a permanent I2P-address" below)

Changes in command line arguments:
1) New command line options are added:
-i2psessionname= - name of an I2P session. If it is not specified, value will be "Bitcoin-client"
-samhost= - address of the SAM bridge host. If it is not specified, value will be ""
-samport= - port number of the SAM bridge host. If it is not specified, value will be "7656".
-mydestination= - your full destination (public+private keys). If it is not specified, the client will geneterate a random destination for you. See below (Starting wallet with a permanent i2p-address) more details about this option.
-generatei2pdestination - if this option is specified, your client will start for generation i2p-destination only and will close after that.
2) New possible value for "-onlynet" option is added:
3) In all options required IP-addresses now you can specify destination hash: <512 symbols>AAAA or b32-address: <52 symbols>.b32.i2p.xyz

1) All of the rest options (except of described above) is not changed and working as they did before
2) Database-file peers.dat is not compatible in both way, neither from vanilla client to my patch, not from my patch to vanilla client. This file must be deleted (or renamed) before using my patch and before going back to vanilla client. This file doesn't contain any important data, so you don't need to back up it.
3) all the rest of databases is compatible with vanilla client.
4) Full backward compatibility of the bitcoin-protocol! Patched client works fine with unpatched vanilla clients. Changes in the protocol are appeared only if both clients are patched.

Starting wallet with a permanent I2P-address:
By default if you don't specify -mydestination parameter the patched wallet generates random i2p-destination. It anonymizes your work with bitcoin even more than before. Especially if you use a new bitcoin-address for each transaction. But if you want to help network by publishing you i2p-address or you just like some i2p-address you have to run wallet with permanent i2p-address. You should specify a full destination in the "mydestination" option for that. Full destionation consists of base64 destionation (b32-address is not allowed here!) and your private tunnel key.
Run from command line:

Or you may use your config-file (bitcoin.conf):
$ cat ~/.bitcoin/bitcoin.conf

The first 516 symbols (SVAxp......~vaZfIQP6AAAA) are public key. This key is a base64 i2p-address (aka dest-hash). The rest symbols are private key. If you want to use some i2p-address you need to know its private key. If you publishes you private i2p-key anyone can use your i2p-address. So keep in secret your private key. This program don't send your private key, it uses your key only to open i2p-tunnel. A simplest way to generate your own I2P-address is running the patched client with option -generatei2pdestination. A generated address with a private key will be shown and you will can copy it to your config-file and publish it.