Just for completeness I want to share with you that in Mexico with Prodigy Movil (the major hotspot operator, a Telmex subsidiary) I also have problems with my iPad from time to time.
The workaround I have found is to clear cookies, navigation history, cache and then I can log in and browse with no further issues.
The failure is very consistent in 3.2 but have not tested with 3.2.1.
Maybe it is an interaction between the iOS, Safari and the captive portal platform they use.
I am waiting for the iPad to be officially available in my market (from this Friday, I think) to file a report with the provider tech support but I would not hold my breath for a fix.