Here is Schluters explanation of how Ditra works:-
Uncoupling Membranes - Uncoupling Theory Part 2 - Schlüter-Systems
Here's the UK spec sheet:-
Broken Link Removed
On page 5 of that document it states:-
6.Immediately after the Schlüter®-DITRA 25
matting is installed, the tiles can be set
using the thin-bed method and using a
bonding tile adhesive that matches the
requirements of the covering.
So it doesn't state if polymer modified is or isn't an issue.
It's interesting how there seem to be different specs between UK and USA as Peter mentioned earlier.
It would be very interesting to hear Schluters clarification of this - it used to be big no no (polymer modified) when I fitted Ditra, but things change.