This bug bit me after DP3 and I reported it to Apple. It is still present in 10.7 and I have tried all the suggestions here and a few others. It is occurring on a MacBook 4,1; installing 10.7.2 actually made it worse. I have tried clean installs, PM resets, prefs. keychains, antivirus off—you name it. Nothing reliably fixes it.
Today I was working at a major telecom maker where they build wireless measuring instruments and have a first class fast wireless network using the latest Buffalo AirStation Base Stations in every room. It bit me soon after I logged in with the WEP password; the connection was OK at first and then disappeared after 30 minutes or so. When it disappeared the MB had lost its DHCP-provided IP, netmask, router and DNS settings. After that, ALTHOUGH the wifi icon was showing the strongest signal ( I was 2 meters from the BS) I could not connect and the MB had an internall provided IP address. It ws as though the MB and BS had stopped handshaking but the MB "thought" it was still connected.
Embarassing to say the least and not a good ad for Apple in this company's tech section.
Next I visited a legal office where they use Aple Base Stations and it connected perfectly first time and stayed connected for 1.5 hours.
So is the reason some of us are seeing this and others not due to an eclectic mix of Apple model, airport chip and base station wireless controller?
I am not sure, but with 6000+ views, I guess that more than 100 people posting here have this problem and Apple needs to get the 10.7.1 patch released.
When it happens next, I'll try running Wireshark to see what it shows.