Techwomyn

Airport documents/fixes dominate Apple's tech articles recently
http://docs.info.apple.com/article.html?artnum=75185
Maybe some of these will apply to your situation.

You are still asking for an older version???? I posted a link in my earlier post

Tolman--
incoming=true is correct, and eliminates any upstream blocks like the firewall as the problem.
To eliminate user pref problems
http://docs.info.apple.com/article.html?artnum=151701
shows how to create a new Admin user. This will virtually isolate all your former prefs, so if you install a fresh LW in this new account and see the same problem, then you know the problem is outside of your Home folder.
You are using the ethernet port, right? Some ADSL equipment uses the USB port, which is inadequate on OSX for the number of connections needed for p2p apps.

btw--good luck getting an accurate answer from tech support. At my ISP, even the head of the help desk didn't know for months that the internet guys were experimenting with p2p traffic controls.
(CISCO router's NBAR features, and a trial of Sandvine software). No malice was involved--hell, they were ready to dig up and replace my cable--the helpers were just unaware. jum, et voilà, trap_jaw and the LW devs helped me get the data I needed to reliably show the problem.

Murasame--the upload bandwidth controls don't affect the download speeds, and the Tip of the Day in the LW Help menu has all the details on the OSX firewall.