This post follows my earlier one citing what to do if your phone can browse the web but not sync email.
The following instructions are for Windows Mobile 6.0 and were taken from my HTC S620 - other handsets are very likely to have exactly the same options.
If you are travelling outside your country of residence then the chances are your phone will roam onto one of a number of possible carriers. There are cases where the carrier your phone automatically selects is not the best choice given your contract and requirement for services such as GPRS. To force your phone to use an alternative network do the following:
Select "Start, Settings, Phone, More, Networks" and then click on "Manual" from the pull down menu titled "Network Selection". Click on the "Menu" button followed by "Select a Network". At this point your phone will take a few moments to figure out which network providers are available based on network coverage. Select the alternative network carrier from the list if there are any others listed.
If you are a Microsoft employee from the UK then you need to select "AT&T" from the list while in Seattle/Washington State.
Once you're done with your trip then I'd recommend changing the setting back to "automatic" otherwise your phone will keep trying to use the network you chose even when in other geographies.