Hi Matthias,
I'll admit that project sounds pretty cool - and I don't want to discourage you from working on it; but I suspect that implementing that sort of functionality in Wireshark might open a giant can of worms, legally. (Especially since MS now own Skype's developers). ;)
Anyway, for getting started with writing dissectors, I'd recommend looking at the documentation in
http://anonsvn.wireshark.org/viewvc/trunk/doc/ (especially "README.developer"), and reading the source code of existing dissectors. When working on new dissectors, I tend to take one of my existing ones, and modify it accordingly, in order to meet the needs of the new protocol in question.
All dissectors are written against a "lowest common denominator" variant of C (C89? C99?), and the EPAN APIs, to ensure portability, and consistency.
I hope that helps,
