Lately, I've been getting a lot of emails which are completely blank. There is no body to the message and there is usually no subject. The only thing in the email is an attached PDF file, with filename that seems to vary. I've been suspecting that it's a virus or spyware of some kind. I haven't looked at the specific PDFs I've been getting, but it appears to be this Peachy Worm.
Fortunately, this worm seems to be pretty benign. It only affects people who are running the full Adobe Acrobat application, not the Adobe Reader that most people have. Also, it only appears to show a simple 'Peach' game on the screen, so it doesn't appear to do any harm.
We definately need to keep an eye on these though, now that we have virii spreading in PDF files too. If you see an email like I describe, just delete it and don't run that attachment.