This has been an ongoing issue with MBPs for years (there was a giganto 300+ post thread over on the apple board about this - it predates Leopard coming out). Plenty of blog posts all around the web on things to do to fix.
Mine was immune to it initially - then some wireless card driver firmware update was released late summer/early fall last year - it started dropping connection after anywhere from 2 hours to 30 seconds of use. Sometimes it was so bad I gave up and plugged in the ethernet.
I literally spent all told about 50-100 hours of my time (spread out over several months) trying to fix this. Even bought a new Atheros wireless card from China, by the time it arrived 10.5.6 had come out, which incidentally fixed it.
One thing I can recommend - don't look for advice here. I wasn't able to find much help. Hang out on the Apple board (flat out - the people there are far more technically inclined) + do an exhaustive google search.