Ryanair seems to have taken the drastic step of implementing captchas in the booking process in order to stop Online Travelagents using screen scraping techniques to book tickets on the site. It will be very interesting to se how this turns out, other companies that have tested captchas in a similar way have reported declines in sales as a good captcha sometimes is hard even for a human to solve. If it is implemented late in the booking process the problem of lost sales is probably minimized (as customers have already taken the time to fill in personal details etc).
The main problem in implementing captchas in this particular way is that it is very easy to circumvent for a programmer (regardless of the strength of the captcha). It remains to be seen how quick the screen scrapers are to adopt to this or if they chose to start following Ryanairs terms and conditions of use.