I've got similar issues -- following the 10.6.5 update, my wireless reception became intermittent, reported signal strength went from the full 4 "bars" to 3 and then 1.
Following advice on the web, I've sorted it out temporarily by resetting the SMC (powered down, battery & cable out, press power for 5 seconds). It held for a day or so and I then had to do it again. Now I'm trying to shut the wireless down before letting the machine sleep, and it might work.
I've updated from 10.5.x to 10.6.0, 10.6.1, etc. and never had this before.
My hardware: MBP 15 (2.4GHz), early 2008.