When tiling a shower surround, how do you transition from backer board to greenrock?
Backer needs to go up at least 5 feet and extend a couple inches out a tub surround and used in all parts of a shower.  It only needs to be in the wet areas.

For tub surrounds I normally put two horizontal pieces of backer board (goes 6 feet over tub top).  Tubs are usually 5 feet in the US and backer is sold in 3x5 pieces.  I then put one piece vertically on each side (going out a few inches) plus a partial piece to meet the 6 foot back wall.  So I need to buy 5 pieces for a typical tub surround.  

You meet your drywall from above and side from there.  Just a normal join.  I use mortar to fill backer to backer joins and drywall mud for back to drywall pieces.  I really really suggest tiling over your backer to drywall joins too since 1/2" backer isn't as thick as 1/2" drywall.  The tile coverage makes it easier and looks better.  You don't need to backer behind your toilet or anything like that.  

Edit - I would also note that I almost always have to add 2x4s to allow me to attach the side backer boards and allow for a firm place for drywall to meet. If everything in the room is "down" then I would actually put up 2x4 after backer - so I can make sure it meets in the middle with no cut.  Cutting a 1/2 inch of backer board isn't worth the time!  

Also I like a 2x4 right outside my tub external too - these are there a lot of time but not always.  Obviously I would have to put this in pre-backer install.  This is more for attaching your drywall.