If you've got an Intel wireless card (especially 802.11n adapters), I highly recommend using the latest drivers from the kernel wireless project.

I have an Intel 5100 and was experiencing lots of lost connections, especially under any type of load. Not only that, but it would typically max out at 60Mbps. Since updating to the latest stable drivers, I now get around 130Mbps and haven't had a single disconnect, even when pushing a total of 2.4GB at 9.9MB/s to a server on my LAN. It also seems to associate much quicker, but that might just be my new love for the card.

As a note, these same drivers are available in Ubuntu via the backports package, but I don't recommend using this as it includes a lot of other drivers, and for me, I experienced kernel panics when using them. I also note that uninstalling them doesn't remove them from /lib/modules/2.6.x/updates, so you get to have fun figuring that out on your own.

I finally got my issues resolved by completely removing all files from /lib/modules/2.6.x/updates and following the build/install directions for the new drivers.