I have to admit that I have no experience fighting spambots and don't really know how sophisticated they are. That said, I don't see anything in the jQuery article that couldn't be accomplished purely on the server.

To rephrase the summary from the jQuery article:

1. When generating the contact form on the server ...
2. Grab the current time.
3. Combine that timestamp, plus a secret word, and generate a 32 character 'hash' and store it as a cookie on the visitor's browser.
4. Store the hash or 'token' timestamp in a hidden form tag.
5. When the form is posted back, the value of the timestamp will be compared to the 32 character 'token' stored in the cookie.
6. If the information doesn't match, or is missing, or if the timestamp is too old, stop execution of the request ...

Another option, if you want to use the traditional image CAPTCHA without the overhead of generating them on every request is to pre-generate them offline. Then you just need to randomly choose one to display with each form.