Almost offensively easy to use, Webthumb's API works off a simple xml query that you can make from almost anywhere. It's very easy to set up a lot of the finer details as well, including how the thumbnail is cropped, delay before taking the thumbnail and of course the size -- indeed, you could just have a full view the site as opposed to a thumbnail.

The service is free to try out, with a 100 free credits a month (one request counts as one or more credits depending), though additional ones are quite reasonably priced. And while I probably wouldn't recommend generating a thumbnail each time from the flash, there's certainly no reason you couldn't set up a cron job using flash or VRML or whatever the kids are into these days.