I had this problem as well. It coincided with Opera not working either, so I assumed it was a network issue. First, check your APN / TCP settings.
Options / Advanced Options / TCP
T-mobile
APN: wap.voicestream.com
Username: wap.voicestreamgprs.com
Password: <blank>
Cingular (not verified):
APN: wap.cingular
Username for APN:
wap@cingulargprs.com
Password for APN: cingular1
WAP Gateway IP: 66.209.11.61
WAP Gateway Port: 9203
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If that fails, try pulling the battery for a few seconds.