If you're referring to the offline providers (the one used by the Local Database Cache Wizard in VS), it has no client provider for SqlExpress/Server. The provider combo is SqlCeClientSyncProvider/DbServerSyncProvider. There used to be a sample SqlExpress
client sync provider but was pulled out from the download site already.
If you're using the collaboration providers, SqlSyncProvider (which works for Sql Express, Sql Server and Sql Azure), the provider can be used as local and remote provider.