Found: Vista drivers for the Toshiba M200 tablet.
There has been quite a lot of discussion from M200 owners about whether or not Toshiba was going to support Vista on this model or not, including this detailed post from Scott Hanselman.
When I first got the tablet, I set up a Google blog search RSS feed for “m200 vista” and it paid off exactly as I had hoped. On the forum GottaBeMobile, someone posted that the drivers were spotted on the Swiss Toshiba support site [link]. You don’t have to actually read the foreign language (German?) page, the dropdowns are in English or product-name-eze.
And it looks like everything is here, video drivers, “value added package”, etc. The biggest problem by far — the tablet not coming back from sleep — seems to have been addressed with the new video driver.