One other point that I would like to add is the following. I have noticed that since upgrading to Exchange 2K3 the amount of time that it takes to expand these DL's has nearly trippled. One thing that was noticed in the DTrace is that apparently we now make calls to AD to verify the validity of the AD account, see as follows:
CLDHelper::ExpandDistributionList - Member has associated AD account
When Journaling against Exchange 5.5 this entry is not in the DTrace. Please provide some details if possible, but if that is the case, are we making calls to AD to make sure that the address being cached does exist?