How about developing OWA Premium for Firefox only? That way it would work on all platforms and be quick to develop (joke). That said, the lack of Firefox (and Safari) support is truly lame, but not a large surprise from MS. I understood the lack of support for alternate browsers in OWA 2003 (the state of the browsers was much different at that time), but this is 2006!
Google is rocking on the cross-browser support (and doing all those fancy AJAX things) -- just look at the calendar UI. IBM is talking up Firefox / Linux and Mac support with the new Notes products. It will be interesting to see if Google releases a hosted integrated enterprise version of gmail / calendar... You better believe that it would rock multiple browsers.