Platinum is Microsoft's code name for the next version of Exchange after 5.5. 5.5 itself was codenamed Osmium, a rare metal with a bad odor.
Platinum is next in the periodic table.
Most of the people who know anything meaningful about Platinum are constrained from saying anything about it while it's in beta. However, at MEC 1999 Microsoft is planning to pull the wraps off of it and show it off. Stay tuned for more details.