I've had the same problem, I've had my Diamond since it came out months ago and I've tried several times to get picmail working. Just last night I came across this: http://forum.ppcgeeks.com/showthread.php?t=22509
[the first post has a lot of details about the app.] Read through that and you can download the program and try it out, I really liked it and donated the $5 through paypal like the creator suggested and I was surprised by how quickly I was able to register and get the full version up and running [and you get a lifetime license for that donation]. So far, I've gotten sending picmail working just fine with this program. Receiving is another matter, and it's an issue that seems to only be on the Diamond and the Touch Pro. Typically on Sprint phones that can't get regular MMS, they'll send a regular txt message with a link that you can click on and download the picture, they were able to get this program to intercept those txts and automatically download the picture like regular MMS is supposed to work. However, on the Diamond and Touch Pro, it does not send a txt with a link to the MMS since the native Sprint picmail app should be working (but they just can't seem to get it working at all!). They have wrote on their site [http://smms.vision5.com/
] that they are working on that problem. In the meantime, in the app you can just click on your inbox and it'll sync all your recent new picmails right in the program, you don't need to go through the Sprint picmail site and you can view the pictures, download them to your phone, reply to the messages, etc. It's just a matter of how often you get picmail which would determine how often you should check for anything new. Alternatively, if you tell people that when they send you an MMS make sure they send a regular txt message telling you they sent the MMS, you'll be able to go into the program and check. This app seems way easier to use than the native Sprint one [if it even worked] and seems like these guys can get more things done in their free time than the developers over at Sprint who should have had this problem solved by now. It's also quite fast at sending the pics [and notifies you when it's done sending] and you can set if you want the pictures resized before being sent.
Hope that helps.
Feel free to contact me if you need any help,