Love the new conditional fields. Except they are not working in IE6, IE8. Have not tested in IE7. Of course they are working fine in every other browser.
Please download the 2.6.6 package again. There was a JQuery change last minute that broke this feature in IE. Sorry for the inconvenience, but it is now resolved.
angekay - if you have the latest build that includes the fix, but your conditional fields are not working, then you probably don't have them setup correctly. There was never an issue in FF to begin with... only IE. Please watch the updated videos on custom fields (updated yesterday) and see how to properly setup everything. Also be aware that there are other changes applied regarding how and where the fields are used, which is why the upgrade instructions said to re-save all custom fields and also re-save event setups in regards to field settings. If you continue to have an issue after checking all of this, submit a ticket and we'll help you out.