.Net FX 3.5 fully downward compatible ?

[Muji-FightR]

I just saw the new .Net FX 3.5 and realized I didn't even have 3.0 :O
Anyway, I was wondering about its downward compatibility, whether it's possible to install 3.5 only and still be able to run all .net apps ranging from 1.0 to 3.5...
I was once told it's better to install all frameworks (1 and 2 at that time) but I don't really like the idea of installing like 5 versions of that framework...

Does anyone have a clue about this ? I already googled a little but as always it just confuses you even more, some people say install all of them, MS says 3.0 includes 2.0, but I also read that there are compatibility issues with 1.1 apps running on 2.0... bah

BTW If anyone of you guys can find sn.exe I'd be glad if you could send it to me. It's supposed to be integrated into the framework but I couldn't find it, no luck via google either :-/