"SIM Card Not Allowed"(Unbranded Treo 600 using AT&T Service)

[mkbomb]

Happy Thanksgiving, all!
Bought a used Treo 600 from ebay. Didn't come with a SIM. Put in my AT&T 3G SIM, phone says "Sim card not allowed. Your phone cannot be used with this SIM card". Spoke with AT&T Business class reps(2 different people) as well as AT&T store folk, and got varying answers, from "No, the 3g sim won't work", to "Yes, it will work, just give it 30 minutes", to "Just update the firmware", to "Ask for the SIM that says "GSM"-he assured me that EVERY AT&T store offers the "GSM" sim as an option for all older handsets. Yet another rep told me that the 3g sim is backwards compatible with pretty much any handset, but that the Treo 600 REQUIRES a "32K" SIM ONLY!!
Am I out of luck, or can updating the firmware(which I have NOT done yet) help? It'd sure be nice to have a working phone for Thanksgiving. I'd be real grateful to anyone that could help me here.

Edit: I did search the forums, to no avail. Also, the Phone is Unbranded. This morning, I installed Palm's "Cingular 1.16 Updater", In hopes that would work. When I try to run it from the phone after hotsyncing it, I get "This updater is not compatible with this phone".