There are already a few sites up that test for the heartbeat extension and also try to actively exploit it if it's enabled:
https://www.ssllabs.com/ssltest/
I can confirm that they successfully detect a vulnerable Linux host.
If anyone happens to have or temporarily arrange for internet-facing hosts/vCenters/other vSphere products, they should give it a try.
Edit:
https://github.com/justfalter/heartbleed/blob/master/jared_stafford/heartbleed.py
Tested the above script against a 5.5 host and it reports vulnerable as expected, while <5.5 does not.