NASA are holding a press conference at their Washington, D.C. headquarters at 2pm EST (That's 7pm GMT/UTC, check Celestia for your local time zone...

No leaks yet, (which is good) but the BBC are reporting that it may be related to current water on Mars, and apparently everyone out in JPL is very excited about it.
I'm betting its either current water in the trenches (why else fly everyone over to D.C. for a press conference) or fossilised life in the bedrock Opportunity's been looking at recently.
Either way, prepare to be amazed!

If, like me, you don't live in the U.S and don't know whether your local news station will be carrying the press briefing live, you can view NASA TV on realplayer here: http://www.nasa.gov/multimedia/nasatv/index.html
More details on the pre-announcement can be found at http://www.space.com and http://www.spaceflightnow.com