Exchange AddressEntry.ID not the same when querying from a 2K user and a 5.5

We have built a system that uses exchange as the user profile data store...
The web interface connects to a Exchange 2K box which in turn has all the
exchange 5.5 servers in the organisation conected to it. We are able to
extract the EddrassEntry.ID for every user and I am wanting to use this as a
global unique identifier for the users, since you can also use this number
to query other properties for a user.

The Problem:
When a user logs on that has an account on the exchange 2K box, the id
retreved for users are not the same as(for the same user) when an exchange
5.5 user logs on. It seems that the two exchange servers are using different
address books. (Yet the entire organisation uses a single central global
address book, which is replicated to all the servers in the organisation.)

Is there any way that you could get the same id for any user when users from
any domain logs in? Is there another UID that can be used?

Any help would be greatly apreciated.