] "Initial analysis would suggest that Sobig.F is a
] mass-e-mailing virus that is spreading very vigorously.
] Sobig.F appears to be polymorphic in nature. The address
] is also spoofed and may not indicate the true identity of
] the sender," a MessageLabs statement said.

I've been getting bounce messages today from gateway anti-virus systems claiming I sent them this virus. I haven't. I use MacOS. I've you've been getting similar messages, this is the reason.

World squirms as Sobig returns | CNET News.com