Adobe has finally released an update to mitigate a critical vulnerability that first became public knowledge back in January.
The security update is for Acrobat Reader 9 and Acrobat 9. You really shouldn't be on an older version at this point unless you have some peculiar application compatibility issues. Fixes for versions 7x and 8x of both products will reportedly be available by March 18, and an update for Reader 9.1 for Unix is expected on March 25.
If you use other PDF readers or writers, you may also need to apply an update for that particular software. Be sure to check the vendor website or run the update feature from the tools or help menu.