Whee! There's a security bug in Adobe Reader (you know, the application that lets you read PDF files). It applies to you - yes, you. Windows, Mac, Linux - all vulnerable. I expect that browser plugins are vulnerable, too, since they use the same engine.
Even worse, it's a "Day Zero" exploit - no security patch is available for you to protect yourself. Exploit code is circulating, so there's a chance that the Bad Guys might have a "party at your place", and you know what a mess they'll leave behind. Fortunately, it's super easy to protect yourself:
Step 1: Start Adobe Acrobat Reader. For Windows users, you'll go to your Start menu, and Acrobat 9 is probably there.
Step 2: Go to the Edit menu, and select Preferences.
Step 4: Click "OK". Bravo - you're safe. No pwn you long time for you (at least via PDFs, at least by this method).
UPDATE 16 May 10:43: Patch is available now.