Shortly after I upgraded to OS X 10.5.6 on my MacBook Pro, I was unable to stay connected to my home 802.11n AirPort Extreme network. After a bit of research, I came up with the following solution. Not sure if all the steps are necessary, but it worked for me. So, if you're having problems, give it a shot! If these steps solved your problem (or not), please let me know in the comments.

1. Turn AirPort off.
2. Repair disk permissions via Disk Utility.
3. Disable Back To My Mac and log out of MobileMe.
4. Install Combo updater (available here).
5. Reset SMC (instructions here).
6. Boot and turn AirPort on.

Hope this helps!

–Greg