Just thought I would share to all of those who will be unlocking an UK Orange Omnia 7 handset to work on another network (it could be the same for a locked Omnia 7 on T-Mobile or other networks) - you WILL NOT be able to change the MMS settings on the phone, i.e. you will not be able to Send (and more than likely receive) any MMS (Picture messages).
Microsoft, in their wisdom, have decided that carriers can effectively lock out certain functions, like MMS settings. There is currently no way of accessing MMS setting on Windows Phone 7 - HTC created an app called "CONNECTION SETUP" I think, which effectively does the job for you, however, I haven't found a way to download it from the Marketplace when not on an HTC phone. I have even tried to send the ZUNE link from a HTC phone so that I could call the app up directly from within the marketplace on the Omnia, however there are no install options.... BUM!
Even resetting the phone with your chosen network simcard installed will NOT auto load the MMS settings (as you might expect). Even your network APN will not be configured for you, but that's ok because it is available for you to create.
You will have to wait until either Microsoft update the OS and make the settings available OR some kind person writes an app similar to the HTC one.
It's not a deal breaker, but those thinking of unlocking for their network may think again if MMS is important to them!