Hi,
I have exactly the same setup as the previous lister: 1st GEN iPhone, 2.1 firmware, with the same settings in Swirly. Although I had Swirly working fine before (Firmware 1.1.4), when I upgraded to V2.1, it changed my IMEI number back to the original, and MMS quit working. AS yet, I haven't been able to get it back to 1.1.4 so that I can change the IMEI number, so it quit working. I went through the steps at the beginning of the post (change the SIM card back to another phone, a RAZR), and then tried to send am MMS from that phone, and it didn't work then either. I kept resending the MMS on the other phone until it finally worked (it took an hour or so, but it finally did). I then took out the SIM card from the RAZR, and put it back into my iPhone, and now it works again. I believe that the key is to get it working with your iPhone SIM in a different phone (call AT&T if you have to), and then switch the SIM card back to the iPhone. I'm also fairly certain that you have to drop your "iPhone" account with AT&T, by telling them you are no longer using it, and get them to change your account to a "regular" account (you'll lose the visual voicemail, but that's it).
FYI, if you stick a small pin into the hole at the top of your iPhone, it will eject the SIM card...