> there may be newer better methods, but
> the type-in-the-letter hacks i know of are somewhat
> labour- and resource-intensive on the spammers, so i think it might 
> be worth a shot.

The method used to defeat that technique has been to have the bot pass 
the type-in-the-letter image through to a pron site. Users at the pron 
site type in the keyword in order to see the next image. Then the bot 
takes the results back to complete the challenge at the first site.

Insidiously clever, co-opting humans to do the jobs which are too 
difficult for the computer!