UIs need to know whether the streams in a call to a contact are, or will be, mutable, so they know whether to present separate audio and audio+video call buttons (for Google Video Chat contacts, for example) or just a generic call button.
The attached spec branch adds a StreamedMedia property and a StreamedMedia capability. See also <http://people.freedesktop.org/~wjt/telepathy-spec-immutable_streams/spec/>
The Gabble implementation in Bug #22589 has implementation errors, but is sufficient to prove that this is implementable.
The patch on Bug #22934 undrafts ImmutableStreams.
Fixed in 0.17.28