reCAPTCHA box not being aligned correctly

You need this css rule:

#recaptcha_area { margin: auto}

I'm posting an updated solution to this question, as the accepted solution does not work with the new version 2.0 of Google's ReCaptcha.

Correct formatting is:

.g-recaptcha div { margin-left: auto; margin-right: auto;}

The new ReCaptcha uses a single div class called g-recaptcha and a number of unidentified and unclassified div children. This is probably the cleanest safest way to get the widget to center properly.