Try posting this on the bathroom forum. A lot of tile installers hang out over there. You can get answers to all your questions from the people who do it all the time.

found a PDF of mfg's instructions

Here is a link that might be useful: durock installation instructions

This stuff can be cut with a carbide blade in a circular saw. Be sure to wear eye protection and a dust mask. Hopefully you don't have too many pieces to cut as Hardy backer will eat up saw blades.

You can also score both sides with a carbide cutter and snap straight lines.

Hardibacker is easy to score and snap as well.

http://www.jameshardie.com/backerboard/homeowner/hardibacker_installation.php

Also much easier to tape and mortar the joints as you tile to prevent any high spots at the joints.

Just curious about this.

One is supposed to glue down the cement board, but who actually does this? I'm in the DC area and I have yet to encounter a job where they weren't just screwing it down every 8 inches or so. Do any of you guys actually glue it down?

It can be glued down with thinset.
It is not really so much to fasten it in place, but to make sure there are no gaps between the backer and substrate.
In some cases the cement board is even fastened down over an isolation membrane to prevent cracks in the substrate from passing into the cement board.

Backer board can be scored with a utility knife and snapped just like sheet rock, then use the utility knife to cut the mesh.

Ripping up the current flooring/underlayment can be done to get the finished floor height lower---or if that is not a concern, just apply thin set, screw the backerboard in place, just tape the joints with the correct fiber tape(no thinset necessary), let stand for 24 hours so the thinset will cure(there are epoxy based thinsets with a faster cure time), apply the thinset and the tiles.

There are fiberglass-reinforced-cement backerboards and there are fiber-cement backerboards. Which type are you proposing to use?

You also need thinset under the CB. And have you checked your deflection?

We talk about this a lot in the Bathrooms forum, but you may just want to go over to the John Bridge forums for the expert advice. They will help you through all phases of your project.

Here is a link that might be useful: John Bridge Forums

If you have a recipricating saw, buy a blade that is designed for tile. It has grit on the cutting side and works well to cut the board.

If you have asbestos flooring, it would be best to put the cement board over it, if the flooring is in good enough condition. Better than releasing asbestos into the air.
Norm

Asbestos tile is not considered friable (capable of being crushed in the hand).
It is not really a very signifiacnt hazard and all but the most alarmist states allow homeowners to remove the material.