BUG: Microsoft.SharePoint.SPFieldLookup.LookupList (Documentation)
The SPS 2003 documentation claims that this property “gets the GUID of the list to use for the lookup.” This is incorrect; it returns a string. Furthermore, this string is not guaranteed to be a GUID; it may be the name of a list. In either case, you'll have to write some pretty defensive (and rather ugly) code to get around this.