We have created and released a new module Boost Captcha to allow static page caching of pages with forms with captcha without running into the captcha session timeout errors. By helping boost cache more pages the module will allow for better performance without compromising on spam protection.
The module loads the captcha in the form through an additional AHAH request once the user starts filling in the form. So if the user does not fill in the form the captcha will not be reloaded thus avoiding the additional bootstrap. The additional bootstrap will only be used when users fill the form. This is as opposed to the option of completely not caching the page at all.
Typically when pages are statically cached for very long periods of time (longer than 24 hrs) the captcha session for the generated captcha in the form will expire and causing the form submission to result in a "CAPTCHA session reuse attack detected" error.
Although the module is named boost captcha which is where we have run into this problem the most, the module should work well with any static page caching scenarios where the pages are to be cached for very long periods of time.
You can read more about the working of the module at the Boost Captcha module page.
Do try out the module and let us know via the Drupal issue queue if there are issues - https://drupal.org/project/issues/boost_captcha