Actually guys, some good news on this front. You CAN do it now. It is just not obvious. The following general "recipe" is from the book "iPhone The Missing Manual".
To send an email with a photo attached to another cellphone (essentially, send an MMS), you simply need to know the "magic address" to send to for the network that cellphone is on. Here is the small list that was in the book. It assumes that the cellphone you wanted to send to is (123) 456-7890 (substitute your intended destination wherever you see this number in the list below). For each case, this list shows the wireless carrier your intended party uses, and then the address to send your email to for that carrier:
Alltel:
[email protected]
AT&T:
[email protected]
Sprint:
[email protected]
T-Mobile:
[email protected]
US Cellular:
[email protected]
Verizon Wireless:
[email protected]
Virgin Mobile:
[email protected]
To make use of these simple, the book recommends that you simply setup the correct version of the above as a secondary email address in your address book for the person in question. Then it is easy to select it from your contacts list on your iPhone.
Just address your email to any one of the above and you should be "good to go". BTW, I haven't tested this, so I cannot vouch for it.