I never had any problem with such sites (e.g. T-Mobile, and many others). I access them exactly as I would on my laptop: Connect to hotspot, then fire up a browser at any URL (on the N800 I usually just hit 'tableteer' because it's right there in the bookmarks). Then I'll get redirected to the login page, and after going through whatever hoops they want me to go through the other networking services (e.g. email, skype, gtalk) start working. As with the laptop.

IIRC some people reported some problems with a particular type of hotspots+N800.. was it BT Openzone or something like that?

And lastly, there's some software out there (mentioned here on the forum several times) which can automate access to sites with login screens.