Updated first post with a little more info.
For you users with 4.3 OS...I'd strongly suggest that you get the 4.5 OS from one of the stickies in the top of this section, just navigate to the one meant for your device model and FOLLOW THE STEPS!!!
Some users report 4.3 OS and MMS being compatible, some report otherwise. Either way, you're missing out in many fixes/updates/additions/changes/etc if you don't upgrade...even HTML e-mail, which you know you want.
Alltel has not released the 4.5 OS, but other service providers have...and they will work just fine on your device.