We have a mixed Solaris/Windows environment. We went through quite a bit of research. regarding a unified name service/ authentication method. We felt it was too soon for LDAP. There are other non-LDAP tools. (Is Windows Active Directory really following LDAP standards or has Microsoft "embranced and extended" it?)
Anyway we settled on Solaris PC-NetLink (now let me say right here NetApp does not support Big Endian DC's), BUT you can use the "prefdc" option to point to a standard Windows BDC with PC-NetLink being the PDC. The is some manual transfer of account info, but in theory Unix account info can be transferred to the PDC.
Also you don't say if on the Unix side you are using NIS. If you are, Microsoft has "Services for Unix" I think its at 2.0 version that will let a Windows box be a NIS master. It has some tools for moving users accounts between name services.
I too believe LDAP is the answer, but I don't think all vendors are up to speed yet. I am keeping an eye on Sun/iPlanet (Netscape Alliance) because I think thats where Sun's support for LDAP is being developed.