The only driver I've actually checked this in is the Orinoco one, but it reports link status by printk()ing something. This is fairly useless. I can't see any reason for the drivers /not/ to report this kind of thing in a useful way, and it'd certainly be nice to have userspace be able to do DHCP when I move within range of an access point rather than when I bring the interface up. You could probably hack something by looking at signal strengths, but it doesn't sound ideal.
In further news, I successfully hacked the BIOS of my X40 and now have a working wireless card. http://www.srcf.ucam.org/~mjg59/thinkpad/bios.html has some details.