For 7.0 we added a HasRead property to the Forum Thread SHOW request so you can specify whether to mark the thread as read or not when getting it:
telligent.com/.../29604.show-forum-thread-rest-endpoint.aspx
We also added a mark as read endpoint for forum threads (not documented yet) at the following url: ~/api.ashx/v2/forums/{forumid}/threads/{threadid}/read.{xml/json}
But nothing really available prior to 7.0.