Here are some of good options for what to do under these circumstances:

PM a dev or mod using these forums
Log into IRC and PM a dev
Email a dev (if you can find an email address)

One thing that's not such a good idea (just in my opinion) is to post it in the public forums and flag it up in the title for everyone to see, because there are plenty of people out there that won't hesitate to use the bug/exploit when they're being outplayed, or even just don't feel like trying too hard for a few games, and we end up with a lot of multiplayer games that get ruined.

Since the last exploit was posted I've had it used against me in multiplayer games several times, which is a bit annoying because it makes a mockery of any skill to the game until the the exploit is fixed. Luckily the exploit in question was caught by the devs early enough to be fixed in 1.03, but in future we might not be so fortunate.