Wifi problems Snow Leopard/MacBook Pro

Lately my MBP has been having problems connecting to certain wifi networks. The symptoms vary:

does not see the network at all when my other wifi devices do
connects, but with no internet access
proxied/browser login required does not load login page

Yet it does work with some wifi networks, both secured and unsecured, and for the longest time, everything worked. It's very annoying

Any suggestions? is there some way to 'reset'?

I did muck around with some settings file so that Internet Sharing would work with my Kindle when my MBP has Ethernet. But that was quite awhile ago now and is probably unrelated.

That happened to my Mac in my last few days of running Snow Leopard before upgrading to Lion. Mine usually didn't see any available networks and when it did, it'd say "no internet connection" also. A reboot usually fixed that, although I never got to the bottom of why that happens. I'd perform a hardware test just to make sure there's nothing wrong with the wireless card in your MacBook and if everything turns out okay and the problem persists, I'd re-install Mac - not a fresh install where you'd have to format your computer, but just re-install/repair it.