Recently, my co-worker shared the following information.  Just wanted to pass it along...Thanks Matt!

When CRM 4.0 was first released you could update CRM data via the Import wizard. But then, for no apparent reason, Microsoft took this feature away with UR1.

Well, they've added it back, apparently with Update Rollup 9. Now, if you export data to Excel it will include a hidden column "A" that contains the GUID for the record. You should not remove or modify column “A”… just make your changes to data in Excel, then save as CSV and re-import to CRM. You'll now see an option to "Enrich existing data in CRM" during the import.