Maybe not
I find it to be more likely that the reason Apple did NOT include MMS is because Safari does not support the protocols (Yet).
I'm not an engineer, and I have only read a little into the standards, but my speculation is as follows:
There is a whole list of protocols that are required for a device to be "MMS Complient" by 3GPP group. Many of these standards are implemented in the Acid3 test, through SMIL, and thus, when a mobile browser passes Acid3, it will also support MMS. From there, it would be simple for Apple to impliment MMS themselves, probably from within the current SMS client.
Safari 4 beta is already passing Acid3. I am not much for rumors, and people can feel free to call me wrong, but I would not be surprised if Safari 4, AND iPhone firmware 3.0 WITH MMS all came out at macworld. Officially. From Apple.
Further, I suspect that once the iphone supports MMS, the carriers simply need to change a binary bit in their servers, and suddenly we all have MMS support from them too.
Remember, I'm not an engineer, I'm not a software designer, and my knoledge is thin and these are merely my suspisions and speculations. Further, it makes sense to me that if Apple didn't have the technology at the time, and they don't have the ability to impliment it, they wouldn't admit it, because that would tarnish their image. I think it also makes sence that they would only be investing as much time and money as they are into passing acid3 if it surved them a greater good. MMS support would be a greater good.
Just my $.02