Interesting .. i am trying the exact same thing and had to find out they give you the api but actually let you decompile the source.
So where is the point in not putting it on sourceforge?
The problem i find is that apparently you can only create complete new items and let them implement the required interfaces.
Since the source code is not public (or atleast i nowhere could find them) there is only 1 way left and that one is of course not the official one: decompile.
When i started i was full of joy and pleasure and willing to learn everything, but unfortunately they keep there secrets well hidden i guess.
The community could be so much better if we beginners could get a good tutorial but its nowhere to find.
Now if you search in this forum you will find this post:
[tutorial] How to decompile IC2 with MCP
So actually have to decompile ic2 to get started than read code and try to understand whats happening because the api descriptions are more than lacking and no help at all.
What i really would love to see is an sourceforge repo or at least a tutorial HOW to get an item working with the ic2 api.
Greeting from a really sad minecrafter 
We plan on including example code sometime soon(tm)