Are there any known instances of malicious security breeches exploiting the Heartbleed vulnerability? Or is there at present only a known vulnerability, albeit a very serious one?
There's several proof of concept examples out there, and researchers have been able to use these to steal private keys and user login credentials, among other things.
I have come across a blog post detailing how to use a python script to automate fetching user session information from a vulnerable server. I have also heard people were able to steal Yahoo login credentials in plain text.
So yes - There are attacks that work and potentially anyone could be using them right now.
Well, for the purely malicious, those tend to be secret until the results are leaked.
For an attack that actually found someone's password, there is this Twitter example of a Yahoo.com user's login name and password from Mark Loman, which is pretty clear that at least 1 user's login credentials have been seen by someone using Heartbleed.