Is there a migration path from Exchange 5.5 to Titanium? If so, what is that path?
Yes, there absolutely is. Right now it is looking like it will be a swing (a.k.a. move mailbox) migration. You won't be able to do an in-place upgrade. However, I cannot say any more than this for two reasons: first, I am under a non-disclosure agreement with respect to Titanium (TI). This means that I can only talk about things that are public information (such as the information delivered at MEC 2002 this year). Second, Titanium isn't a released product. Generally, I avoid making definitive statements about unreleased products, as anything can happen between now and release.
Eventually, there will be a lot of Titanium content available at Microsoft's Exchange Server section. If you have a contact at Microsoft, you might ask them to submit you for inclusion in the TI beta program. You can also send an e-mail to BetaReq, although there are no guarantees with this alias.