There is no support for 64-bit in Office Communications Server 2007, however the word out there is saying that it will be supported in the upcoming version of OCS, called Office Communications Server 2007 R2. (Coming this fall)
UPDATE: (July 28.)
A comment posted about the upcoming version of OCS 2007 R2 release date on this blog, states that the new release will be 64 bit only….. However this is not confirmed.