To make the reCAPTCHA stuff more explicable, I finally figured out one place to put text that helps explain it.
In line 46 of recaptcha-wordpress-2.7/recaptcha.php, I replaced:
<div id="recaptcha-submit-btn-area"></div>
with:
<div id="recaptcha-submit-btn-area"><font size="2">Please enter the two words you see above in the blank text box and click submit to submit your comment.</font></div>