I'm busy to get the CAPTCHA module for Drupal 6 past Beta and RC state and one of the last issues I'd like to iron out for CAPTCHA 6.x-2.0 is removing the preprocess operation from hook_captcha. For more discussion and details see. Feel free to tune in.
In attachment I have a patch that should make the reCAPTCHA module compatible with this API change: it replaces the usage of the custom hook_captcha-preprocess with standard Drupal Form API stuff (mainly a '#process' callback).
Note that this change is also backwards compatible with the "old" CAPTCHA API (CAPTCHA 5.x-3.x and 6.x-1.x), so nobody gets hurt.