But it looks like "all other devices" (PC, Nokia 5800, etc) are using different operating systems than Linux and thus different implementations of connecting protocols. I can confirm that N900 (and generally few older Linux distros) have problems with connecting to some routers (I have an old router at home which sometimes behaves similarly with my N900 like you described - and it also sometimes just fixes itself after few days or after doing some strange operations... But it also works the same way with old Slax-based Linux distro - [offtopic]in case anyone's interested - it's my custom distro based on ancient 4.X line which I'm too lazy to update to newer version and newer custom packages :P anyway, Slax 6.X connects fine everytime I run it from my usb stick[/offtopic]).

So I would also support the version that router or, in other words incompatibility between some old or very cheap routers and some versions of Linux connecting implementations could be the problem here.

P.S. All words stated in above post are only my oppinion and I had not invastigated any further, I can just confirm that some routers have problems with Linux clients, but work fine with Symbian and Windows clients. Anyone who is more familiar with and has more knowledge about the problem, feel free to add something what I had said or negate and proove me wrong