Fiona, one of the devs on my team VSCore, has put together a really nice sample on how to implement IAccessible for a Custom Push Button.  If you’re using standard Windows controls, you get MSAA implemented for free.  However, if you’re using custom drawn controls, it is imperative that you implement MSAA (aka IAccessible); otherwise, the control will not be accessible to Assistive Technologies.

There's a lot of documentation on MSDN how to support MSAA, and now you have an actual sample of how to implement IAccessible!  Just point one of the MSAA SDK Testing Tools (Inspect or AccExplorer) at the custom button for a proof of concept.

This sample follows the MSAA SDK guidelines on how to implement IAccessible for a push button.

The sample is located on GotDotNet at

http://www.gotdotnet.com/Community/Workspaces/Workspace.aspx?id=4b5530a0-c900-421b-8ed6-7407997fa979

Enjoy!