Solving automatic properties compile problems when migrating ASP.NET 2

I've recently migrated/adapted a web site built on ASP.NET 2.0 (.NET Framework 2.0) to .NET Framework 3.5. Oh the goodies of .NET Framework 3.5 : LINQ, lambdas, automatic properties and so on. At first I just set the build option to .NET Framework 3.5 : Everything fine and dandy till I ran the site : WTF? I mean it is set to use the .NET Framework 3.5...