GSM Association's document IR 61 provides their guidelines for WLAN roaming [PDF format]: A European colleague wrote to point me to this download, which was created by the cell standard group GSMA, members of which run the world's GSM networks. The document describes the procedures for handling authentication in a uniform manner for interoperator roaming.
Having followed these documents for a while now, I'm amazed at how the Wi-Fi community has jumped on the WISPr bandwagon without apparently realizing that telcos have been addressing the Wi-Fi issue slowly and steadily for quite some time now, even to the point of drafting entire integration and business models (complete with user and third party roles) that provide a comprehensive (even if somewhat biased) road map to full "roaming" functionality (based on EAP-SIM, of course, so you implicitly become a GSM user).
These documents (and many others like them) can be taken as more hints that the consolidation phase of the Wi-Fi market is arriving, meaning that smaller players (despite their technical merits and closeness to the "grass roots" efforts people like to read about) will soon have to compete with big, experienced companies that can leverage existing resources (customer care, etc.) and business processes.
It's not just about the technology (or the authentication, billing and whatnot) - it's how Wi-Fi can be tied in to the entire GSM value chain. And since pretty much every European mobile operator is a GSM operator, this will most likely be another reason Europe will deal with Wi-Fi in a very different way from the US...