I've noticed that coming from T-Mobile USA I'm unable to send any MMS to a user on Verizon Wireless. Every other carrier they go through fine, tested it out on 4 different users in different regions of the US(Arizona, Florida, New Mexico, North Carolina) over the past 2 weeks and none of them received it, or even a notification. They receive them just fine if they're sent from my BlackBerries, HTC Win-Mo phones, or the Nexus One so I know it's a fMMS issue somewhere. No idea what it would be though.