Syncronization is not impossible, but it can be a lot of work. It depends on how smart you want your syncronization rules to be. For example, if two users modify the same record, but different fields, should the syncronization script use the newest record, or somehow merge the two? If you want to merge them, always keeping the newest data on a field-by-field basis, then you'll need to keep a modification log and use that to set the data on a field-by field basis.
I have no way of knowing what FileMaker's plans are, but this is certainly not a common need. Most deployments use networking to share databases, either on a local network or a wide area network. Anyway, I think you'd have the same kind of syncronization issues to work out with any other DBMS.