Not supported and not technically possible on a 64bit server. There is one .net assembly dependancy which is hard coded to 32bit so it won't run under 64 bit asp.
Sad for Small/Medium business. We've gone from using 1 box per site (80 users) for Exchange 03 / LCS 05 to needing 3 for Exchange 07 / OCS 07. So much for consolidation!