I had an issue with the Nokia repos originally when trying to install stuff using wifi, but I noticed it was OK over the cellular network. I tried using nslookup to do a DNS query on the Nokia repo - it took a very long time to give an answer over wifi, but did it immediately over cell.

Other machines on my wifi network were able to get DNS lookups for the same nokia URLs without issue. I set my N900s DNS servers manually, to point at my ISPs DNS servers, and now its all fine (although there are still different issues with extras-devel).

Having said this, I have often had DNS issues with my ISP, and my Draytek Vigor 2820vn router is also known to have DNS issues itself, so I normally point most of my machines at another linux box running dnsmasq as their primary DNS server. I know that the N900 uses dnsmasq itself, and pointing it at the other box running dnsmasq didn't help, so I pointed it straight upstream.

I know this might not help anyone else, but then again I thought it worth mentioning.