This has got to be one of the most, ahem...creative, solutions I have seen for captcha generation:
https://account.uk.miva.com/Advertiser/Account/login.asp
View source, and search for "SecureText" to find the section of the HTML that contains the captcha "image." Then scroll to the right. Keep scrolling...keep scrolling...
Brillant!
I'm just surprised they're not using floated divs or an enterprisey java applet.