On a recent project, a reviewer mentioned that SQL Server Express should never be used in a corporate application because companies would never install a database on the client. This is wrong headed because of two important trends: IT Centralization and ClickOnce Deployment.
As part of an organization that had many hundreds of Access applications to migrate to a new Wide Area File System (WAFS) infrastructure I can inform you on the pain of herding a plethora of unmanaged databases. As the tools to centrally manage IT improve, it will become more and more untenable for a corporation to accept unmanageable databases.
I say unmanageable databases, because the second trend, ClickOnce Deployment, not only speaks of centralized deployment but also of unprivileged user installations; really, self-service installations. This is the "trick" of Express Edition that older school architects are missing, in my opinion. You get the deployment and database management ease of an MDB file but the power of a real database. With Expression Edition being virtually part of the operating system (something you can enforce with centralized IT), internal teams can play with MDF files just as easy as they did with MDBs, but, when it is time to grow up, the switch to a centralized server is painless.
A final trend is simply inevitability: JET and MDAC are dead; Long Live SQL Server Express Edition. Deal with it.