I have this problem when I have an e-mail that has attachments sent to me from work and I try accessing them over the 3G network. It works fine on a LAN, but over 3G it just doesn't happen. I can get the headers no problem, but the minute I try and download an e-mail that has an attachment it seems to hang.
Another possibility if you're using 3G is that it's actually connected to the 2G network (if signal strength is low). If you've got a lot of e-mails in the box it can take forever for the headers to come up.
Just a few thoughts.