I hadn't had time to test the plugins yet, but today I saw there was an update for Jetpack. So, after installing the update, I decided to test my contact form, hoping the update had resolved the error issues.
Unfortunately, here are the results:
Whether I'm logged in to WordPress or not, I get this error now after submitting a contact form:
http://screencast.com/t/qD9G06ZXawj
And the email notifications have quit working again. :-/
I will attempt to test each of my plugins next.
If this continues I will probably resort to a different contact form.