If you’re one of the many people being faced with Mac wifi disconnects recently I have a quick tip that may help. Restart your Mac and then hold down the keys Command+Option+P+R during the restart. You will hear the Apple boot sound, and then the system will reboot again. Once you hear the second Apple boot sound let go of the keys. This will clear the NVRAM/PRAM. Your Mac stores certain settings in a special memory area even if it is turned off. On Intel-based Macs, this is stored in memory known as NVRAM; on PowerPC-based Macs, this is stored in memory known as PRAM. More information on this can be found HERE.
I originally thought this was an issue with Mac and Cisco IOS so I upgraded IOS on a few Cisco Aironets but the problem didn’t change. I even used a different make of access point but that didn’t make any difference either. Additionally Windows systems, cell phones, and other devices were unaffected so I had a pretty good idea it was Mac related. What made the issue more complicated was it didn’t always happen. Often the Mac would startup and then be ok. After a few minutes to hours of use you couldn’t ping the gateway or access point. Regardless the Mac reported as still being connected to the wifi. Turn wifi off then on and the problem may or may not go away for a period of time.
Updated 2/24/2013
The above resolved most of the issues but some Macs still had an issue resuming the wifi connection coming out of sleep. Perform the below and it will resolve that issue.
1) disable wifi
2) delete the file Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist
3) enable wifi and select your favorite network again