One thing to keep in mind. The cracking tiles are NOT the problem. They are a SYMPTOM of the problem. You need to locate the source of the movement, or you'll never get the tiles to stay in one piece. One thing you can check yourself right off, if this is over woodframe construction (and not concrete slab) and I hope I'm wrong-- something many sales people over look (so they can make the sale) is the fact that when using natural stone, the floor has to be literally TWICE as strong as it does for any other kind of flooring, including ceramic tile. What you want to do is go downstairs into the basement and take a look at the joists. Take measurements of how thick and tall the joists are (2x?), as well as their on center spacing, and their unsupported length. Take those measurements and go to the following web page and plug in the valures in the appropriate places. It'll tell you whether or not your joisting is strong enough for travertine, or any other kind of natural stone:

Here is a link that might be useful: John Bridge's Deflectolator

Thank you Bill. I'll have to look at photos when the joists were exposed, or perhaps our contractor knows measurements -- this bathroom is on the second floor, over the living room. The contractor put Durock on the entire floor. They also had to level the floor outside the shower with concrete (?) over the Durock. However that concrete leveling doesn't extend over the whole floor, in fact I think it may have tapered off in the area where the tile cracked, if my memory is right. I hope we took pictures of the leveling before the tile was laid. Could this be a factor? Meanwhile we'll look into the joist measurements and whether the joisting is strong enough.

The leveling wouldn't be a factor, unless they didn't use the primer, but if they didn't use thinset between the Durock and plywood, that could be a factor.