Ok - found a site that seems to be making the effort with captchas, but.... they also seem to have ballsed it up.
The site in question is our old friend Paypal. To see what I mean (and I couldn't find way of linking directly), have a look at www.scrounginggit.com and hit "donate" (Or any other site using the paypal "donate" button that you know of).
Inside the pop-up is a very usful link " listen to the security characters." - which either opens or prompts download of a .wav file. A damn sensible solution, encapsulated in a potentially inaccessible way....I am perplexed. Anyone think of a good reason that link couldn't be put next to the original capcha?