I guess I should consider myself fortunate that I don't ever have to program for that - every application I have written has been for an environment where sight is assumed.

I'd be careful about those assumptions (disclaimer: people pay me for accessibility work :-)

For government or government funded sites in the UK, US and in other countries accessibility is a major issue - contractually or legally depending on locale. For business sites it's becoming a potential legal/PR minefield.

The advantage over using an image is that it can't be scanned by a bot and read.

Yes it can. Automating a web browser and a screen grab program isn't hard. With a little more effort they can just parse and interpret the HTML directly.

The question is - is it worth the effort for somebody to do this on your site.

The WAI have a nice working paper on the topic Inaccessibility of Visually-Oriented Anti-Robot Tests for those who are interested in the topic.

Personally I have found heuristic server-side solutions much more effective. For example:

• Require an response from the user via email
• Keep an eye out for registrations coming from the same IP/domain
• Keep an eye out for registrations with similar data
• Feedback forms with "random" names and a tracking ID to make them do a lot more work to automate the submission.
• ... I'm sure you get the idea...

Depending on your application it may be worth thinking how much a captured registration is worth in the currency of your choice, and then thinking about how many registrations a minimum wage worker could make on your site in an hour. If the math comes out the wrong way you're going to have to rethink anyway.