Not Juvenation, and it was more an invasion of privacy than pretending to be me, but I've had an issue at a previous workplace. After I found out, I changed my password (at home!) and deleted all the internet and temporary files on my work computer. Then I never accessed it from work again.
As someone elase said, it sounds like someone crossed the line. I learned from my previous experience to never consider anything on my work computer personal, and also to stick with the established computer use policy. (Having an iPhone makes that much easier to do do!)
I can understand enforcing rules or checking for security/policy issues, but some people really do cross the line... and you simply have to take precautions againstit, unfortunately. :(