The XDA Trion is an OEM version of the HTC TyTN, the 3G, Windows Mobile 5.0 based PDA phone from HTC. Both these models are almost identical, save for the "QWERTZ" instead of "QWERTY" keyboard on the XDA Trion. Other than that, even their prices are the same—$899.
The phones have quad-band GSM, 64MB of storage, and a microSD slot for expandable memory. For connectivity, they support GPRS/EDGE/HSDPA/Bluetooth/802.11b/g compliant. There's also a 2-megapixel phone and a secondary camera on the front for 3G video calls, something that isn't popular yet here in the states. Since it's GSM, you should be able to pick one up at the $899 price, and use it on one of the GSM providers here.
If you've already got a previous generation HTC device such as the T-Mobile MDA or the Verizon XV6700, there may not be a reason to upgrade. The software is the same on all these devices. You're just paying for the 3G support, higher quality camera, much faster processor, and video calling. If you don't need any of that, you can stick with the older model.