It's been a long time since this has happened but I almost got lured in a virus. I got this UPS message saying that a package I had sent was undeliverable as addressed. They instructed me to print the attached invoice to pick up my packet from my local UPS office.
Since I recently shipped a package, I fell for it. I realized just as I clicked that UPS would never send an attachment for something like this. It would be in the body of the message. Too bad my mouse didn't hesitate as quickly as my brain but thank Zod this happened on my mac. Downloaded harmlessly to my desktop and thusly tossed in the trash. Whew.
For people who send and receive packages often, it could be an easy one to fall for. I figured it would be worth a warning. Apparently it's a new virus that began circulating July 1, 2008.
http://urbanlegends.about.com/b/2008/07/15/ups-virus-warning.htm