Visual Studio Productivity Power Tools Version 10.0.20427.1, updated 10 May 2011, is creating a <solution>.sln.docstates file.

What is purpose of the file?

Should I be checking it into version control?

Solution:

Here is what Microsoft have to say about it:

The .docstates file new in this
release of the Pro Power Tools. It is
used by the Document Tab Well
extension to persist and hydrate the
state of the floating tab wells. This
information was being stored in the
hidden .suo file but we
had to move it to a separate file to
fix a set of crashing bugs (timing
issues). In the next release of the
Power Tools we will hide this file by
default.

http://visualstudiogallery.msdn.microsoft.com/d0d33361-18e2-46c0-8ff2-4adea1e34fef/ (Q&A tab, “Is .sln.docstates file created by Productivity Power Tools?” discussion, post by Chris Dias)

So effectively it is a supplement to the .suo (solution user options) file, and like the .suo file it should not be checked into version control.