I'm running Ubuntu Karmic 9.10 on a laptop and faced some difficulties with WiFi
unstability (frequent disconnections) but things have greatly improved since I
moved from the default "network manager" to "Wicd" another network manager
included in the distribution, I still have disconnections (maybe due to
interferences with other local WiFi networks working on the same channel) but it
reconnects automatically and never disconnects when downloading something.