One of the most popular mobile accessories that I have heard about from others who travel quite a bit is the Novatel Wireless MiFi. I personally have not tried one out because they have been available on Sprint and Verizon Wireless and I don't get a good enough signal where I live with these carriers to pay for another mobile data service. Novatel Wireless just announced the MiFi 2372 HSPA device that is optimized for North American GSM networks and worldwide usage with a GSM carrier. There have been unlocked ones you could pick up from an importer, but this newest model also has other features integrated into it.
Novatel has opened up the Novatel Developer's Lounge so that developers can download the MiFi applications programming interfaces and programmer guide to create new interfaces for the device.
There are no detailed specifications yet available on the frequencies supported with this newest MiFi (kind of important with an announcement such as this), but I imagine it will have 850/2100 MHz support for the US so that AT&T customers can use the device. It would be great to see T-Mobile's 1700 MHz frequency supported as well. Since it is a global solution the 2100 MHz frequency for other parts of the world should also be present.