Thanks for your response Kristian. I fully understand that you guys need to define scope for your project to ensure you deliver the best possible experience to the largest chunk of the OWA userbase.
With regard to Firefox and IE coming closer to supporting standards in a consistent approach, I do applaud the IE team for going down this path.
You mentioned your framework for deciding browser support within OWA, specifically pointing out that usage is a key determining factor in making decisions about inclusion of a particular browser version into your support model. This model of support is quite dangerous, however I - and projects I have worked on - have used it in the past.
I raise the browser-share model as a dangerous model because usage share can be skewed when you cripple the experience of non-supported browsers. For instance - I use Firefox (or derivatives thereof) 99% of the time, however I always visit OWA with IE. I'd rather not, but the crippled version of the product is so much less desirable than the premium version that I find myself with no choice. Choice is the key word here, and I believe (no doubt some will disagree) that given the choice, OWA browser share may be significantly different to that currently reported with OWA 2003.
Hopefully, when IE6 support finally goes the way of IE 5.5 support, your team will be free to embrace standards and do away with browser specific hacks (a whole other topic).