Despite the fact that the WiFi+3G models are connected to a 3G wireless network, such as AT&T in the US, these connections are data only and do not provide traditional mobile / cellular features such as voice calling, SMS, and MMS.
There is, however, a very simple way to send MMS messages from your iPad to friends and family with iPhones and other phones that receive MMS messages. This does not require that the destination phone has email support. By emailing your picture to their phone and not their email, they'll receive it like a normal MMS message.
1) open a photo on your iPhone
2) select to email the photo
3) using the table below, enter the phone number of the person you want to send to, followed by the email suffix for their provider
|Service Provider||MMS Email Prefix|
So, for instance, if you wanted to send an email to a Verizon user who's number was 8175551212, you'd send to email@example.com.
There may be third party software packages that provide additional ways to send MMS messages from the iPad, or it is possible (however unlikely) that Apple will decide to add expanded messaging capabilities to the iPad. In the meantime, this is a perfectly viable way to send MMS messages from the iPad.