The time has come to evaluate MySQL as a database backend for .NET applications, in most cases replacing SQL Server rather than Access. Admittedly, Microsoft has done quite a bit over the past few years to stop the defection to open-source databases, offering SQL Server 2005 Workgroup Edition for small businesses as well as the free Express Edition.
However, there still is no platform independence and open-source sexiness included with any of these offerings.
Luckily, the entire MySQL-related stack is now maturing to make the switch tempting: MySQL from version 5 has support for stored procedures, triggers, views and (compiled C) functions. The MySQL Connector/Net 5.0 offers a ADO.NET driver to simplify development without resorting to ODBC. This connector can even be integrated with the Microsoft Enterprise Library which might entail ironing out some CLS compliance issues. To work with Visual Studio 2005 you can install the MySQL Visual Studio 2005 plugin to design MySQL objects right from the Server Explorer! So if you're considering a cross-platform app, the Mono Project gives you the development platform and MySQL provides a high-performance database backend which is maturing to enterprise-readiness. It looks like C#/CLI could really become the "new Java"!