I finally bit the bullet and formatted my ASP.NET CAPTCHA server control as a CodeProject article. This version of the control has a few significant improvements over the last version:

• Optimized with use of HttpModule and Cache objects
• Removed ViewState for Captcha text (this isn't secure, doh)
• Added .CaptchaChars property for specifying characters used in random CAPTCHA text.

This has been through quite a bit of testing and refinement, and should be considered final-- for now anyway. If I update it any further, I'll do so through the CodeProject article, so leave comments there if you have any.