Like Matt posted, we (at Carlson School) use and have good results from 
the  "Detect content within a hidden form element" technique mentioned in 
the WebAIM article. We also moved everything clearly internal behind a 
Central Auth page.

It is pretty easy to implement the hidden form element techinque and has no 
impact on the end users (which we like).

reCaptcha does provide for the social good of digitizing old books, newspapers 
and magazines, so I would recommend it if you are set on using a Captcha.