I've had this problem with all my Macs at home, although it would never occur with other Wi-Fi access points. This leads me to believe it has something to do with the wireless access point itself.
I was never able to fix the problem, but it went away after I bought an AirPort Time Capsule, which now serves as my wireless router. It has an extremely limited feature set compared to a real router, but I haven't had connection problems once since implementing it.
Do you use a bluetooth keyboard? My wifi flakiness seems to be much less of a problem when my lid is open and I'm typing on the builtin keyboard than when the lid is closed and I'm using the Apple bluetooth keyboard.
It's so intermittent that it's very hard to diagnose, but that's one thing I've noticed.
I was never able to fix the problem, but it went away after I bought an AirPort Time Capsule, which now serves as my wireless router. It has an extremely limited feature set compared to a real router, but I haven't had connection problems once since implementing it.