I did try at a friend's house and my N900 was able to connect to other protected WLANs fine, just not to mine until I changed the channel from 6 to 3.

I wonder what caused it to stop working after 3 weeks.

I changed my WLAN channel from channel 6 to 3 and that allowed my N900 to catch my WiFi for 2 days without a problem.

Yesterday night same issue again, I could connect to my neighbors unprotected WLAN but not to mine's.

I changed the WLAN channel again, this time from 3 to 4 and the N900 was able to connect to my WLAN afterward.

It would be good to know what causes the deadlock to happen

May be you could install and run wifieye to check the wifi strength around you, graphically?

sorry to dig this one back up. but its suddenly started happening to me. funny thing is it seemed to coincide with intsalling qtirecco which some1 else also mentinoed. the connection continuously goes dead. it claims to still be connected but any attempts to access websites or ping another host on lan fails. i have to disconnect then connect again. and atm the connection then works for only a few seconds.
im using wpa.
i see "deauthenticating by local choice (reason=3)" in dmesg a lot tho im not sure if thats when it hangs or if thats when i tell it to disconnect.
maybe the irreco daemon is somehow interfering with wpa_supplicant?

I have the same problem, the conection only works for a litle time. I`m using WPA2(PSK) with a encore gaming router

I have the same (or similar) problem with my n900. However I have experienced a gradual degradation in its wifi functionality. I purchased it November 2010, for an upcoming trip to the US. Here is how it has degraded:

1st two months - phone worked great, used it in the US (Im from Australia) and was able to download, torrent and practically everything (order flights, net banking) all at a reasonable speed. Wifi settings were default (powersave max and 10mw wifi).
2nd two months - whilst back in Australia, the powersave feature started to cause problems. The browser would hang after ~5-10 minutes internet browsing. Reconnecting would sometimes solve the problem, however on some connections it simply would not authenticate (public connection at my university). Turning powersave off solved the problem (intermediate didn't work properly either).
Last week - all wifi functionality has stopped working correctly (though gprs is still fine). The wifi icon shows its still connected, however no internet functionality - error "Problem loading page - Server not found".

To fix it I I have tried:
1) different combinations under the 'internet connection'
2) turned it on and off
3) deleted saved connections and reconfigured them (it sees the wifi accesspoint without a problem)
4) I will try reflashing my phone, however even if that works (some posts suggest it won't) it isn't an acceptable solution as it'll just occur again in 3-4 months.

PLEASE HELP! Anybody who is technically/software savvy, please give a definitive explanation or suggestion on how to permanently fix the wifi problem or even whether it can be fixed. I love this phone and since N9 is canned (N950 might also be) I fix is mandatory. I'm guessing its a software issue, however the degradation has been sort of linear so it may also be hardware slowly failing.