The problem with these methods is that they pretty much stop any blind user from using these web sites, but that's another issue.

A good way to handle that issue is to also offer a sound byte that reads out the word. This is also hard for automated scripts to bypass, and if a blind person is using a computer you can pretty much guarantee that they have sound.

I don't know of any websites that actually implement both methods though. I guess the issue with implementation is that the graphic image is easy to randomly generate using software, but the sound byte is much more challenging to generate!

Anyway, this is getting off topic, so I will stop here...

I guess the issue with implementation is that the graphic image is easy to randomly generate using software, but the sound byte is much more challenging to generate!

Not really. Just create a series of files each with it's own audio for a single letter, then string them all together and send the concatenated audio. We have an application that uses a similar technique to generate real-time navigation information for ships.

PerlJam

Just create a series of files each with it's own audio for a single letter, then string them all together and send the concatenated audio.

But wouldn't it then be easy to generate a program that knows about the signature of each of those letters? The challenge is randomizing the sound of each letter so that it is easy to understand for a human, but hard to recognize by a program.

This is especially tricky since there is already some really good speech recognition software out there.

Perhaps you can overlay the sound of the letters/words with some music, or something else that will not affect the ability of the person to hear the word.