It's good to hear that OWA 2007 will still accept the Alias in the URL string, but it is unfortunate that this isn't on the plate right now.
Is there any way to file a feature request? I know just about every Exchange shop I know would like to make it easier for users to directly open and send mail from secondary mailboxes. I understand the feature for "open other mailboxes" will be there, but I doublt it will be a 100% swap out for directly logging on to the other mailbox. In addition it requires a primary mailbox as you noted which is an issue with my current customer who has sepereated mailbox identified from user accounts for the reasons I listed above (they are valid business reasons that the technology required a work around for).
Thanks for the consideration of a DCR.