To send an MMS to another phone from your iPhone use the number + provider suffix below as the email address.
Provider
Email suffix
Alltel
@message.alltel.com
AT&T/Cingular
@mms.att.com
Sprint/Nextel
@messaging.sprintpcs.com
T-Mobile
@tmomail.net
Verizon
@vzwpix.com
If you were to specify a custom field in each contact with their service provider an application could theoretically use that information and their mobile number to effectively send an MMS elegantly.