Hmm. I actually thought I'd written this up a few weeks ago after fixing my annoying Leopard wireless issues. But perhaps not!

Anyhoo, here's how I solved my wireless connectivity problems on a MacBook Pro running Leopard:

(1) Make sure you have all the Mac updates (including the Login & Keychain update).

(2) Open your keychain and remove all entries relating to your wireless key setup.

(3) Start up in Safe Mode by holding the Shift key during startup.

(4) Once the computer has started in Safe Mode, simply restart again (without holding any keys).

This seems to "flush" something about the faulty wireless setup. It also appears that the signal strength of wireless cards in MacBooks are weaker than a lot of other PC laptops -- and so switching to the Airport Express which allows transmission on the less interference-prone 5GHz band often improves reliability.