I have a very new MacBook Pro, and a few days after switching to AT&T UVerse for Internet service, I noticed that I was starting to get random disconnects.  I had recently installed a software update from Apple, and I thought that might have caused it, but now I think it didn't.

The message I always get whenever a disconnect occurs is:

4/29/10 11:56:15 AM    kernel    AirPort: Link Down on en1. Reason 4 (Disassociated due to inactivity).

These messages appear in the log (Applications -> Utilities -> Console, click All Messages -- filtering on 'kernel' or even 'AirPort' helps narrow them down), for anybody who doesn't already know.

I tried changing many router settings to make these disconnects stop, including the channel, the security protocol (WEP, WPA, WPA2, etc.), and the wireless mode (802.11b, 802.11g, 802.11b/g), because there are many threads on forums suggesting that I change these.  None of them worked.  I tried deleting my network settings on my MacBook and recreating them.  That didn't help.  I even tried changing a kernel setting in /etc/sysctl.conf, which one person did after talking to Apple support, but that didn't help me.

What finally seems to have helped is rebooting the router, as others in this thread have said.  It is going on an hour since I rebooted it, and I have had no disconnects since then.  At this point, my theory is that, a few days after a reboot, the router starts to act "funky" -- behaving in a way that deviates just enough from the standard to cause problems for Apple's software.  But other devices may not notice the difference or may be able to work around it (most likely this isn't an intentional workaround, just a lucky accident).  If this is correct, there's a bug in the router's firmware, and I hope they find and fix it soon.