If you're on a 2G and have upgraded to 3.1 or beyond, you're not going to be able to use native MMS unless you patch CommCenter (see Ewan Hoozarmy's earlier post). Unless CommCenter has been patched, carrier modifications like ActivateMMS2G won't work.
I could be wrong, but I think worst-case scenario of an unsuccessful attempt at patching CommCenter would be having to a restore. I just patched a 3.1.2 CommCenter on a 3GS to enable tethering and it wasn't a big deal for people who are familiar with the file structure.
Not correct..UNLESS you are talking about AT&T users. I have T-Mobile on 3.1.2 and that source from sinfuliphone.com(enable MMS 3.1.2) worked for me.
MMS is working fine but now my battery life isnt as good as it was when i used Swirly MMS but it isnt dreadful either. It may be a bit better than it was when i was using ActivateMMS2G.