I have been using attglobal.net for years -- although at the time I signed up over a decade ago it was pretty good, and I used dial-up often, since then, they haven't added very many countries to their list -- 55 in total. Most places I go will have an access number, but many will not. They also never used to charge "roaming" (it's not in fact "roaming" because it's using their own network, not a roaming partner), but a few years ago they split the world into four zones, and when you use the service outside your "home" zone you get charged.

Otherwise, I use Boingo (which now has a flat fee), and my mobile operator gives me access to both their WiFi and UMTS networks, either at home or abroad through roaming. Using GPRS in roaming is a bit slow and a bit expensive, but more and more places are upgrading to UMTS and prices are coming down rapidly.

I also haven't used a dial up connection in some time now (I think last time was 2005 in Montreal). Nowadays, most hotels have wired or wireless broadband, and if they have dial-up, they at least have an RJ-11 port on the phone. About two years ago, I finally stopped carrying my 6 or 7 phone jack adaptors (Danish, Swedish, Belgian, Russian, Polish, Italian, British, French and German), as well as the small screwdriver I have used to dismantle dozens of hotel jacks around the world in the days before they began accomodating lap-top toting travellers.