Most of us with wireless connection problems have solved them by changing the assignment of IP address from dynamic to static. Give that a shot and see what happens
I seem to be having related problems.
Yesterday I upgraded from Tiger to SL. Now when I try to connect to the AirPort Extreme which is configured to dole out fixed IP-addresses via DHCP by recognising the MAC address, the NIC doesn't get an IP and assigns a crap-address to itself. Even when I manually configure my NIC to the correct router, mask and a valid address it won't connect to the internet. My girlfriend, who also has a MacBook but with Tiger, is having no problems what so ever.
I have to restart the router and wait for DHCP to wake up again and then it works. But it is the same story again when i restart my computer or wake it from sleep et c.
It almost seems like the DHCP-loan won't expire or something?
EDIT:
This afternoon I went to a friend and brought my MB with me and connected top his WLAN with no problems. When I got home again my MB recieved an IP-address via DHCP with no apparent problems. Maybe this "change of scenery" solved things?
I forgot to mention that when I booted SL for the first time the WLAN worked like a charm and then I downloaded all the updates and rebooted, and that's when the problems seem to have begun.
EDIT 2:
It doesn't seem to have solved anything. Also, when I have my MB set up to recieve IP via DHCP and I have shut down and restarted it, thus rendering it unable to connect, I cannot connect to my Airport Extreme via AirPort tools either.
I tried to set up a fixed IP on my MB, but then I can't reach services on the internet, but I can connect to Airport Extreme via Airport tools... WTF is going on?
Also, now I have tried to set the DHCP-loan time in the Airport to a minimum (10 mins) to see if at least that makes me able to have my MB sleep for ten minutes or more and then work at least.