My ThinkPad T43's wireless network connection stopped working. I'm not sure why, but I'm not the first person to have this kind of problem--Regular Joe had a similar problem. His solution didn't work for me, but he offered a good clue that ThinkVantage Access Connections was the culprit.
Here's how I fixed my problem. In Control Panel / Network Connections, click Change settings of this connection. In the Wireless Networks tab, check Use Windows to configure my wireless network settings. Windows immediately noticed that I was in range of my wireless network, and the rest was obvious.
After my first Windows XP reboot, ThinkVantage Access Connections tried to take over again. For good measure, I uninstalled it, and now everything is stable. I am not using ThinkVantage Access Connections any more, but my WiFi works, and that's all that matters.