Not sure if this helps/relates to the issue you're having or not, but on the SDK virtual image I found that apt was setup to try to use an internal Nokia proxy server, which from the outside world is allowed. Log in to the SDK and check to see if there's a file under /etc/apt/conf.d/ that says something like 99proxy (I don't recall the exact file name as I deleted it to fix the problem). If it's there, move it somewhere else temporarily (never delete until you're sure ;-) ), and try running Apt update again. See the other thread for the same issue:

http://talk.maemo.org/showpost.php?p=367317

I doubt this would be the case on the N900 firmware, but never hurts to have a look and see...