GSM notifies you when the other party has merged your call into a conference. It doesn't provide a notification when they un-merge it, but oh well.
For StreamedMedia, I propose adding a new call state flag, Conference_Host: http://people.freedesktop.org/~wjt/telepathy-spec-conference_host/spec/Channel_Interface_Call_State.html#Channel_Call_State_Flags
For Call, I think the idea is to add flags to http://telepathy.freedesktop.org/spec/Channel_Type_Call.html#Call_Member_Flags but I will leave that to someone else.
Here is my spec branch.