Description of problem:
The instant preview of equation graphics no longer works after upgrading from 
lyx-1.3.7-5.fc4
lyx-qt-1.3.7-5.fc4
to
lyx-1.4.0-4.fc4
lyx-qt-1.4.0-4.fc4

Version-Release number of selected component (if applicable):
see above

How reproducible:
always

Steps to Reproduce:
1. Make sure "Preview" box is clicked in preferences
2. Load a lyx document with equations
3. No previews generated
  
Actual results:


Expected results:


Additional info:
The debug info shows lots of occurrences of:

kpathsea: Running mktexpk --mfmode cx --bdpi 120 --mag 1+0/120 --dpi 120 cmmi10
mktexpk: Mismatched mode cx and resolution 120; ignoring mode.
mktexpk: Can't guess mode for 120 dpi devices.

Dropping back to the previous lyx version fixes the problem