We've setup a partnership with a GlobalSIMCard.co.uk - a provider of hassle-free pre-paid SIMs with GPRS data, usable around the world. Clicking the above link, or entering "OVMS" as the code during checkout, will support the OVMS project.
The SIMs they provide have a very low monthly base charge (currently approximately US$1/month), and simple GPRS data rates (normally charged in 10KB increments). They have a web-based interface for viewing charge logs, account balances, and topping up the balance. The biggest advantage is it avoids all the complex data plans and hassle of dealing with cellular providers directly. Ordering is simple - straight from their site they'll airmail deliver to you (SIMs don't weigh much).
The service itself is compatible with OVMS (latest v1.3.5 or later is recommended, as it has the GSMLOCK command to allow you to lock the module onto the cheapest carrier in your area).