How to acceptance test captcha-protected web application functionality?
Yes, the only option is disabling Captcha - for a very good reason. If it's easy enough to bypass it with Selenium, why would you have a Captcha in the first place?
The point isn't to test Captcha with your Selenium tests -- Why spend the significant effort to test a third party tool?
Instead, I'd have a Selenium test that verifies your Captcha is showing up. This validates your Captcha is present and active. After that test I'd turn Captcha off and proceed with your other validation tests. I'd also have a final step of turning Captcha back on and repeating the test detecting Captcha is active. That way you're ensuring you didn't miss something when reactivating Captcha.
Disable the CAPTCHA by using a test code in the input box. If the captcha fails but matches the test code, continue.