Thought I'd add to the discussion because my version of the problem is sort of unique...
I'm using a 1st Gen MBP that worked fine before upgrading to SL. I can connect OK at my home. (though when I first installed SL I did have major problems...some of the "fixes" I attempted for my other problem may have solved this issue.)
However, at the university where I work I only get connectivity in very specific locations. For example, in my office on the 2nd floor of a 3 story building I do not get connectivity (or rather I connect and then lose that connection within 20s). However, if I'm on the 3rd floor of the same building I can connect fine. This happens in other buildings throughout the campus as well...sometimes the higher floor doesn't work and the lower floors do...
It may be worth noting that when I get that 20s of connectivity in my office the signal bars quickly disappear one by one until I lose the connection. Then when scanning for wireless networks sometimes the network won't show at all, other times it will show but have no signal strength and will not let me reconnect.
It should also be noted that my iphone connects to the wireless fine in all locations (as did my 1st Gen MBP before upgrading to SL).
I've tried all the tricks (plists, new locations, etc), talked to the university IT, handed my laptop over to be examined by the university IT, etc. Still no luck. It's almost as if the laptop is picky about which access points it approves of.
Not sure if that was helpful in diagnosing these issues, but thought I'd toss it out there.