Well, don't get over-excited, but there really is such a software! It is called FreeMap and if you speak hebrew and live in Israel, this might be really cool for you. If you don't it still might show you what is possible if you just don't care about the SDK agreements and you warranty ;-)
There is a Wiki that says something about Cydia and sorce, I guess this is about installing their beta version on the iPhone 3G. Unfortunately, my hebrew isn't that good, so I don't have a clue...
Unfortunately, the source isn't working at this time, so I can't even try it.
A demo video can be seen here.