I agree that I shouldn't have had any problems with any channel. I did have it set to Automatic selection of a new channel (I also use Interference Robustness with an 11 Mbps Multicast Rate), and I've never had a problem like this until the 10.6.6 update.
If you search for "os x 10.6.6 slow internet" you'll see that others were having the same and similar problems (for example long/slow startup times). The suggestion to change the channel and reset the base station was something I found posted on the Apple discussion forum in reply to someone posting about the problem and the person who'd had a similar slowdown posted that it worked for them.
As I mentioned, I'm not sure why this worked but my Net access time is noticeably faster than it's ever been now. My guess is that changing the channel and then resetting the AirPort Base Station does something else too. Not sure what or why, but the results were better than I hoped for.
