Swedish company Mobispine has recently launched an iPhone application that will allow you to send MMS messages, a functionality that Apple left out of the iPhone 3G and a service that is very popular in many countries. Their application iSendMMS is currently available to download at the iTunes App Store, although it currently is only operational in Sweden with TeliaSonera.
Once you have downloaded and installed iSendMMS on your iPhone you can send MMS messages outgoing for normal MMS network tariffs, although the application does not yet allow incoming MMS messages. It also has limitations in terms of country of operation, as the application currently only works with the Swedish operator, although the company states that they are working on making the application work in other countries as well (source: networkworld).
With the rumors of a new and upgraded iPhone coming sometime this year, one has to ask the question if Apple will not be making MMS messaging available on the next generation iPhone, given that it was one of the areas they received criticism for when they launched the current iPhone 3G.