Would it be easy to upgrade and scale the Express database later to a full-blown version of SQL Server using replication if I see that my app makes money and is worth the investment paying for a commercial SQL Server version? I think I could probably even find a hosted SQL Server, so I wouldn't have to pay full license fee. But I guess that prevents me from running on a dedicated server...Thats why I am hoping I can move it over later to some scalable solution like Mosso, and not worry about paying for the full license, only for as much as I need depending on the traffic.
MS SQL Server express cannot be used as a replication master, if you are interested in that type of architecture.