Most excellent. Thanks, Matrox!
It looks like ATI cards only support 4320x900, and not the full 1680x1050; following the link tells you:
To access TripleHead2Go Digital Edition's new triple widescreen modes of 3x1680x1050 and 3x1440x900*, you must download the latest firmware utility and PowerDesk software. Before downloading the software, please make sure that your system meets the following criteria:
1. Your system must have a dual-link DVI connector and must be running a Microsoft Windows® XP or Vista™operating system.
2. Your graphics card should be a compatible DirectX® 10 GPU with the latest graphics driver version installed. To check, download the GXM Compatibility tool.
...
* Available only with certain NVIDIA® (triple 1440x900 and 1680x1050 at 57Hz) and ATI™ (triple 1440x900) GPUs.
Still, fantastic news. Let's hear from those who have 3 22" widescreens to see if there are any problems.