Apple released an Airport Update 2007-002 that fixes connection problems that were nagging MacBook and MacBook Pro users ever since the release of the MacBook Pro C2D. Contrary to the previous one released earlier this year.
This update also improves compatibility for the MacBook Core 2 Duo and the MacBook Pro Core 2 Duo when using certain third-party access points configured to use WEP Open System security.
I can't test it personally as my MBP hasn't returned from repair yet, but initial reports from friends are encouraging. Apparently both issues related to connecting to a third party WiFi access point and dropping connections are solved.
[UPDATE] I can confirm it is fixed! My MBP doesn't drop its connection anymore, and connects successfully to third party WiFi AP (tested on Zyxel and Linksys).