Not sure about international carriers, but the iPhone in the UK (probably) wont be able to support MMS until Apple release a software update which enables more than one Access Point (APN).
Here in the UK, each network has at least two access points. 1 is for internet access, and another for MMS use. It's also common for an extra one used for WAP access - but this is not relevant to the iPhone.
If Apple did enable an extra access point then it is plausable to imagine that users would be able to setup a new access point giving cheaper Internet access whilst abroad which is not what the official carrier networks want.
On a personal note, if additional access points were available in the iPhone software then I would be able to use both personal and work email (I have a corporate APN that I need to use for business)
Hopefully all this makes sense. But please feel free to correct me if it is technically wrong. I work for one of the networks in the UK (not O2) and I know this to be the case for my employer.