The ADO.NET Data Synchronization services is now available in CTP. This SDK will make it easier to support the disconnected operation scenarios you run into with Smart Clients. If you are looking for more information or a good demo of the functionality you can take a look at Steve Lasker's blog and specifically his Live from Redmond Webcast.
One thing to note is that there is only a SQL Compact Edition provider and there is no SQL Express provider at this point, which means that if you are using SQL Express on the client you will need to write your own provider or wait for Microsoft to develop one. I would suggest contacting Steve Lasker if want to see a SQL Express provider developed. Although you could write your own, the schema creation may get a bit tricky... I know that a number of the folks that were on this team were from the SQL Server team and they were able to add some system tables to support the schema process.
They also have a synchronization blog available that discusses and demonstrates the SDK.