You don't mention if any of the apps themselves are even open-source, so just a reminder: if there is no source for the app it's not going to possible to port it. You'd be creating a clone from scratch, and that'd be tough.

Anyways, if you have any experience with java the android one might be a good starting point. It is not java, per se, but Google's own version of it, but it should be close enough to require little work.

On the other hand, if the one for Nokia smartphones uses Qt and C++ then it might be simpler to port. But if it doesn't use Qt but for example Symbian's own functions and libraries it'd almost be easier to write the whole thing from scratch.

As for the iPhone app to N900? Well, it's designed for iOS, without any kind of portability in mind at all. I really doubt it'd be worth the time and effort to even try.