Please indicate more clearly whether a session is currently read-only.

Suggestion 1: include a "read-only" toggle button in the toolbar that's on when the session is read-only, and greyed if the client has only been granted read-only access

Suggestion 2: Add "Read-only" to the title bar when the session is read-only

Suggestion 3: Add "Read-only" to the bottom bar when the session is read-only

Suggestion 4: Change the cursor when hovering over a read-only window (e.g. to the PIRATE cursor)

I've frequently assumed that a system wasn't responding when it was just read-only, and occasionally irritated another user by mousing past a Vinagre window and yanking the cursor away from them.  Making it really obvious whether a window is read-only would save me a lot of embarrassment :)