The N900 is not yet a suitable enough platform for enterprise applications until it can be proven that not only does it play with corporate admin systems well (backup, recovery, custom applications), but also that in making it an option that sufficient resources can be given to it and any other mobile device that such an enterprise would support.

Beyond that, those same enterprises would have to engage some serious revamping of their information policies, mobile user policies, and talk with their insurance providers towards implications of users with said data in such an open means.

I'm as much a fan of using the N900 and any mobile in enterprise as anyone (I do now with my N97), but until you can answer all of those issues, the "replace BBs in enterprise" stuff should not enter this conversation.