Can I force a message to be sent as an MMS? Is there a way to force a message to be sent as an MMS?

There are multiple reasons that MMS may be preferred.  SMS and MMS may have different fee structures dependent on plan, carrier, and country.  Additionally, some messaging software may allow for more delivery notification options for MMS than SMS.
As per Broam's suggestion above, I'm posting my comment as an answer. Hopefully some people finds it useful.

If you add a Subject to the message you're making, it gets converted to MMS format. Even if the Subject is empty, at least, that's what I know