The C# library I wrote that wraps SPSS IO library (spssio32.dll) for managed applications is now hosted on CodePlex. It hasn’t been developed much in the last few years, but for what is already there it’s quite useful and has (nearly) all the functions exposed by spssio32.dll. Many developers have already started using it. I recommend you try it out if you’re interested in managed applications interfacing with SPSS Data files.
CodePlex project: http://www.codeplex.com/spss