Ben Hammersley [benhammersley.com] has developed "a widget to create a XHTML Validation Results RSS feed from any page". You can append your URL to his widget, then subscribe to it with your RSS reader. Set it up it refresh automatically at whatever interval you choose. If there's a validation error, you'll get an alert. If not, you get nothing.
The validation is redone everytime the feed is requested. If there are no errors, the feed will be empty. Silence is golden, in other worlds.
A link to the source code is also included on the above page.
That whole concept has some great possiblities beyond just validation.
Simple site monitor?
You mean using RSS to monitor wether there is something new on a site? Well, I never thought of that, but it is so crazy... it might actually work!
Seriously: If you want to "monitor" sites for changes that do not provide an RSS-Feed, a lot of Aggregators have a kind-of "scrape and monitor" functionality already included. But for "custom-requirements" funneling recurring, automatic "tests" through RSS is a nice idea. (Not really new, though.)