I was just about to make a topic about this as I've had this same problem. And yes it is a virus.
It is most definitely not a virus in my case.
This was an account that I only access online via gmail.com, so no outlook or pop access to this account, and no way my computer could have sent the emails remotely.
When I looked in my gmail sent box, the messages were there. Some [censored] piece of [censored] marketer hacked into that gmail account and used it to send emails to everyone in my contact list (fortunately there were not too many contacts since I dont use that account) telling them to buy an iPhone4 from some lame website called dodo.com or dudo.com or something like that.