We're definitely going to use one of the big advantages of the Code-First approach and start writing our entity classes immediately, without worrying too much about the database engine we're going to use.
Truth be told, we already know something about what we're eventually going to use. We won't be adopting a NoSQL solution, as they are not supported by EF yet; we also don't want to commit ourselves into purchasing expensive license plans, so Oracle and the commercial editions of SQL Server are most likely out of the picture as well.
This leaves us with relatively few choices: SQL Server Compact Edition, SQL Server Express, MySQL, or other less-known solutions such as PostgreSql. That being said, adopting the code-first approach ...