>I have been considering to deploy VFP applications running on MSDE or SQL Server Express. May we use those data engines for commercial purposes (keeping in mind its limits) or for development only ?

Like MSDE, SSE is free and can be distributed with your applications. SSE is still in beta though so you probably should wait to distribute that version. Also, according to the SSE BOL, the beta 2 version had an 18 month expiration date. So I expect beta 3 also has an expiration date.