Date: 7/3/2008 7:05 pm EDT
For best results the under-floor surface should be as plane as it can be. Installing Avaire tile over existing ceramic tile shouldn't be an issue but as soon as someone says it can be done then other issues that are questionable arise.
The depth of the grout lines and the wavy tile could pose some problems and I personally wouldn't consider a floor of that nature to be "plane".
You could however easily enough skim coat the floor using a flat steel trowel and some "modified thinset". You would want to skim diagonally so as not to get thumps and bumps in the thinset from the trowel dropping into the existing grout joints as you applied the thinset.