SwirlSpace has just released a MMS tool which allows sending MMS directly from iPhone. As the first revision, this utility can only send images, vidoes are not supported yet, you can’t receive MMS either.
The tool can be downloaded here: MMS v0.1 for iPhone
UPDATE: new version has renamed to SwirlyMMS: SwirlyMMS v0.2
To use it, you need to setup the server and/or proxy first:
Then fill in the title, the recipient:
and finally select an image from your camera roll:
Click send to go.
Source: SwirlySpace