A couple of days ago I noticed that my N900 could no longer update the weather (OMWeather) when connected to my home wifi network. The network connection was fine but it seemed like I had no internet connection (email, browser, ftp, etc); however, the other devices on the same wifi network had no such problem. The strange thing was that my Skype account was logged in successfully on the N900, all my other VOIP/IM accounts reported 'Network Error.' I switched to my 3G network and experienced the same thing: only Skype worked. When I got to work and connected to the office's wifi, everything was fine.
The key symptom was that 'ping-ing' anything would fail on the 3G or home-wifi networks but succeeded on the office wifi-network. A DNS problem. The /etc/resolv.conf had been automatically changed by vpnc to point to the office's dns. I don't know why this problem suddenly happened now, but I've commented out the entries in /etc/resolv.conf to fix the problem (for now?).
