I wrote a managed C# wrapper for the spssio32.dll library that comes with SPSS. It exposes most of the functionality of spssio32.dll for .NET use. All the memory management required to use spssio32.dll is already taken care of, and a small set of common document operations for reading and creating files is even offered in an object-oriented class layer.
You can download the source and visit the forum on the CodePlex project site. [28Feb07 Update: It's released under the LGPL.] [17Apr08 update: updated links to new hosting location]
Note that due to SPSS copyright, the project does not include spssio32.dll, so you will need to get that file to use with this library in order to read and write SPSS files using .NET.