The solution for all wifi clients is to do what iOS and Mac OS X has been doing for years, which is validate internet connectivity when connecting via wifi. If there's a captive portal standing in the way, it pops up a simple webkit view automatically. You don't even have to open a web browser.

It would be nice to have a more formal standard (e.g. supplying an authentication URL along with the DHCP response) but to be honest, this emerging de-facto standard is perfectly serviceable.