Sync Services for ADO.NET and SQL Express on the client
Earlier this week at the SDC conference I gave a presentation about the Sync Services for ADO.NET. Some attendees where pretty exited about this new possibility to sync data between a local data cache and some sever. Especially that they could use pretty much any database on the server as long as there is a managed provider. Now on the client side people where somewhat less thrilled that the only option seemed to be SQLce. In fact only a single person in the audience was currently using SQLce. Now I think SQLce is a very interesting product and could be used as a replacement for SQL Express in a fair number of applications so I suggest you take a look at it. However the good news is that Microsoft is planning on releasing a sample local provider for SQL Express on the client when Visual Studio 2008, Sync Services and SQL Server Compact are released.
Still I highly recommend taking a good look at SQLce as it is more that adequate in quite a few cases and it can make life a lot easier 
Enjoy!