Different linebreak behavior in Windows and Mac OS X -- in Preview only

In Preview mode for Windows under IE7, TinyMCE seems to strip the carriage returns and not replace them with anything on Preview. As a result, the text all runs together.

Post mode for Windows works.

Post and Preview mode both work under FF on Mac OS X.

This is true for Filtered HTML (with P and BR), Full HTML, and, per Install.txt, I've now tried the special input format with no filters.

Nothing is working. Thoughts? Why would this behavior be platform dependent? Could it be the versions of JavaScript used in Windows/IE vs Mac/FF ???

Needless to say, I'd like my Windows users to be able to post...