I too have been experiencing problems with my 2007 Imac conecting to my wi-fi networks for the last three weeks, since upgrading to 10.6.7, while everything else in the house, various windows laptops and my iphone, have had no problems at all. The problem was specific to my own network, as the Imac was able to see those my neighbours' networks, but was unable to connect to mine.
So here are my obvervations on all of the various solutions that I tried.
1) Resetting the PRAM didn't work. There's an Apple note that explains that in 10.6 network settings are no longer reset when the PRAM is reset.
2) Using network preferences to change to DHCP with a manual IP address worked for a bit.
3) Unhiding the network SSID helped, in that the Imac could see it, but then wouldn't connect, unless I physically placed the computer next to the wireless router, (although as time went on even this failed work).
4) Connecting the Imac to a router using an ethernet cable, then rebooting both router and computer worked a couple of times, but invariably the next day the problem would re-occur.
5) Creating a new location in network preferences, and setting up the network from scratch didn't work.
6) Removing my home network from the list of preferred airport networks then rebooting both router and computer worked once, but only once
7) Clearing the list of preferred networks, as in 6 above, then using Onyx system tools to delete the boot cache, then rebooting both computer and router also worked once, but only once.
8) Last night I downloaded the combo-updater for 10.6.7 and re-installed the OS, whereas previously I had updated via the software updater. This worked last night and fingers crossed, I've had no problems since re-starting the computer this morning.
