I want to add my testimony to the fact that you CAN use a non 3G SIM to access UMTS and HSDPA speeds! I think this is just a marketing scam by Cingular and others to get you to switch over to their more pricey plans.

First, a little tip on unlocking. I almost burned my 5 tries first thinking that my Cingular branded Treo 750 was asking me for a SIM unlock code, not a subsidy unlock code. It was asking me for the subsidy unlock code, and second, by forgetting to press the option button to ensure that numbers were inputted instead of letters! Stupid mistake, but I guess I was used to my Palm OS device automatically doing that if it is obvious that numbers are supposed to be used (not sure if Windows Mobile does that as I have to learn it and get used to it).

Second, get the phone to run on the internet normally before messing with any setting (I made that mistake and fuddled around with it for an hour or two before doing a hard reset and trying it again where it then went smoothly).

Third, install the HSDPA hack from URPREY. http://urprey.com/treo/750hsdpahackv2.cab The ONLY thing you need is his CAB. Download it directly into your Treo and double click it to open it in the Treo. No need to download into your desktop or laptop. It will do the work for you. (Thanks URPREY!) (P.S. remember, this is all you need. You DO NOT have to install the the fit4cat Hermes Tweaker.)

Fourth, do a soft reboot.

Fifth, follow Graylion's instructions below to allow your AT&T Wireless SIM to attach to the Cingular HSDPA network (Thanks Graylion!) In the Connections area, under My ISP, click Add a new modem connection and then do the following:

Connections
- General: AT&T/Cingular
- Modem: Cingular GPRS, Access Point = Null, Username/password/domain = null
-- Advanced: use server ip address, use ip header, use specific server 10.250.1.10
- proxy: This network connects to the internet, uses a proxy,
-- Advanced: http server = wap.mymmode.com port = 8080, server = localhost, port = 1080 sock4

Sixth, soft reboot and start flying!

I live in Miami and I've gotten speed readings of up to 1172kbs with consistant readings in the 800s and 900s.

If anyone has any questions, I'll be glad to help. Thanks to everyone who helped me out as well! Now to get used to this Windows Mobile platform and see what nifty programs are you there!

Silent