Hi,
My problem is similar. 3-4 weeks ago, when I received my macbook, I've tried to connect via Airport to the WEP-encrypted 2Wire 1800HG at home. I couldn't get it to work with $ signs, hexadecimal equivalent of password etc. Finally I disabled the encryption from the 2Wire modem's portal, and it worked flawlessly. All this until last Thursday, when I suddenly couldn't connect to the wireless network anymore. The modem works ok, because I can connect via ethernet, and the PCs in the house can connect wirelessly. Also, my Airport card works, because it sees the list of available networks, and I could connect perfectly to a public wifi last Thursday. If I remember correctly, I've installed an automatic update from Apple exactly that day (I don't know which one), so that might be the problem. But the root of the problem seems to be the incompatibility problems that the Airport cards in the new Macbooks apparently have (considering the number of complaints).
In the modem's portal page, the macbook doesn't appear listed as active devices (it doesn't receive an IP via DHCP, even if I try pressing the "Renew DHCP lease" from System Preferences/Network).
If someone from Apple has an official view on this issue, I would like to hear about it. Or, better, if someone had a similar issue and has solved it, please share the solution.
thanks,
M