Some of you may already have a preview version of BizTalk 2006 from TechEd. If so, you may try using MSMQ Adapter on NT cluster. You need to make BizTalk process a cluster resource (there is a checkbox in Admin for that) and have MSMQ service on the same cluster.
In the preview bits that you have there is a trick you need to keep in mind: the clustered Biztalk resource must have dependency on Computer Name resource and enable Use Network name as computer name as check boxes to make it work. Otherwise MSMQ queue will be considered remote and you will not be able to use it transactionally.