I'm sure the carrier negotiations in order to use shortcodes are different in countries outside the US. I have heard that docomo in Japan actually makes this sort of stuff really easy for developers (and that Japan has a burgeoning diy market of cool phone-based services as a result).
What country are you in?
I've done work using the email2sms gateways for both sms and mms. For sms it's kind of a pain because the gateways can't always be inferred just from carrier and number (T-mobile for instance, sometimes uses a nickname that you set up rather than your phone number, so your phone's sms email address ends up something like coolguy@tmomail.net instead of 2125551234@tmomail.net). Receiving MMS is a HUGE pain because the carriers all include the attachments differently. Some carriers include multiple attachments -- some of them are spacer gifs and things like that -- and you have to figure out the correct one. Other carriers, like Sprint, don't even include an attachment, they have a link in the MMS email that takes you to a page, and that page has another link to the picture. I had to use come complicated regex's and multiple wget calls to download the picture. The whole thing was very fragile as a result. But, to Sprint's credit, they are so bloated and slow to change that in 9 months they never modified the way those messages were structured so nothing broke.
I haven't worked on this stuff in over a year, so it may be out of date, but given the glacial rate of change in telco's (esp in the US), I'd be surprised if the situation has changed much.
Again, though, could be a very different story depending on where you're actually located.
http://www.ericsson.com/mobilityworld/sub/open/technologies/...
Its a useful library if the service provider you're trying to gain access to exposes Parlay X access to their network. Very easy to send/receive SMS, MMS, etc from your web application as they library wraps all the SOAP calls for you.
It's quite a racket, they often charge both the sender and the receiver.
This seemed to work but it was hard to guarantee message delivery.