I think that the way to really go about this is to do as one poster said, and replace the brain on an individual neuron basis. Say, wrap each one in a sheath, which "learns" the connections the cell made, and can continue them after cellular death. Of course, once you have this setup, you can emulate them in a computer. Here's the part I think would be interesting.

First, to ensure "immortality" you would want to offload the basic system to a more secure site, and treat the body as a client of the server(your mind). So, you get your head taken off by a shotgun, no problem. You are just sitting in the dark for a bit until your new body comes online. As long as you can maintain a sense of continuity, it's still you, and not a copy(IMO). Obviously, this would take an immense information pipe, with almost zero latency, so this is probably the achilles heel of the whole idea. Of course, if the local client is capable of autonomous existence, and the system is built upon the idea of a constant updating both ways(like some video games do now), then it may still be feasible.

Second, you could take this data stream and record it, or pipe it to another person, say. I once argued that just the ability to share sensation would revolutionize sex. But it would be useful for other things too. You could inject new data into the stream(why carry a cell phone when you can BE the cell phone). Telepresence would be possible. Why waste resources constantly shipping people into orbit when you can ship some "bodies" up and just have whatever expert required use what is already there? And, of course, entertainment. If you think Warcraft is cool now....

Third, I think it would be interesting to see if the human mind, freed from the meat machine we use to think now, could learn to manipulate multiple I/O channels at a time. That is, could you spawn two processes which share memories and consciousness, but can manipulate a body independently of each other? Could you rebuild the human mind to be able to run two or more bodies, and would this affect your feeling of humanity?

So, I see immortality not so much as a basic continuation of this life, but rather a sort of merger with an artificial intelligence. And I would jump at this in a heartbeat. Just the ability to aggregate more and more data would be worth it. Imagine the insight such a person could have if they could truly master all of our current scientific disciplines? Imagine a mind with the creative impulses of a human, the computational power of a computer and the ability to intergrate all of the diverse fields of information we have today? All with a virtually unlimited lifespan?

'bout 100 billion, too big of a number for me to grasp, so I just think of it as a whole fucking lot. I see extremes in temperature the same way. Anything below freezing is too fucking cold.

Off hand no, I do not.  Probably that is easy enough to google but I have another objection.  Heat.

Let's say that you put some unknow number of billions of nano-robots ina human skull.  Even though they are tiny, they will produce some amount of heat as all machines must.  Even if that is a tenth of the heat produced by the structures that they are mimicing, that is a lot of new heat being intorduced into the skull.  So the skull must be reengineered to accomodate the new heat load.

Also, they will need an energy source.  If it is the same sugars that the existing brain runs on, then the body would go into shock from the departure from homeostasis.  If it is something else, you still have to get it there without ruining the delicate chemical balance of the body.

 Answers in Gene simmons, your post about copies reminded me of an "Outer Limits" episode.  Basically, we developed a technology that allowed us to travel lightyears, but how it was done was a copy was made at the destination, and then the operator had to destroy the first copy.  The episode gets funny when they have a mistake and think the person didn't arrive at their destination, so they keep the first copy alive.  Onjce they find out the destination copy is infact alive, they have to literally kill the copy at the space station. 

   How many ways can I say no !??       

Well, it honestly depends on my lifestyle at the time they offered my immortality. If they offered me the immortality transplant thing when I was poor and starving then no. If I was living well, then I would really think about it.

Off the top of my head I can think of a couple of variants. The ancient Greeks had it that Hades was a sort of perpetual afterlife, so it could count as immortality. However, nothing ever happens there so you get to mostly sleep through it. Also, certain versions of Buddhism have and end to the cycle of reincarnation. Ideally, every pass through the world has you getting just a bit cooler but eventually, you reach the point of being 100% cool and then the whole process stops. Your last death is of the ego entirely so there is no more you. However, there is a remnant of all that coolness that merges into and becomes god.