Here's a service on the SL Marketplace with a pretty interesting offer: You e-mail the developer a photo of your real head (no glasses or smiling, please!), and voila, your head becomes a mesh object in Second Life which he'll send to you. (Hopefully just not in a box, Se7en style.) Above, what it looks like as done with a photo of Bruce Willis. (Interesting factoid: There actually is an official Bruce Willis avatar in SL, but it doesn't look half as good as the one above.)
"You can wear the head or make whatever you want with it," Franz Strzelecki writes on the Marketplace, explaining his service, then adds somewhat mysteriously, "Each head has a painted hole for the brain, on the back of the skull." (Emph. mine!) I haven't tried this head mastering service out yet, so caveat emptor and such. Then again, at L$500 (less than $2), it's not a high price to pay to bring your head into SL (if you're headed that way).