An Apple Discussions forum thread discusses a very specific problem affecting a significant number of users with AirPort Extreme cards and more than 1GB of RAM:
"While doing network transfers, heavy downloads, sending files via iChat, logged on to VPN, etc. your network slows down and then eventually completely drops. Your AirPort menu on the menu bar is completely gray and shows no available networks 802.11b or g. The only way to recover from this is to completely restart your Mac."
This particular problem appears to be separate from the various AirPort issues we've been covering the past couple weeks. Lending support to the theory that this issue is related to the amount of installed RAM is the fact that, for many users, removing RAM so that the computer has 1GB or less solves the problem -- connections are stable with no drop-outs.
If you have a Power Mac or PowerBook with an AirPort Extreme card and more than 1GB of RAM, and you've been experiencing these problems, please drop us an email if reducing the amount of installed RAM alleviates the problem.