ARINC 661 is an ARINC standard which defines the interface between the CDS (Cockpit Display System) and Aircraft avionics Systems (User Applications or UAs).
ARINC 661 standardizes the runtime communication between the CDS and the UAs. ARINC 661 also provides a way to completely define GUIs, using standardized configuration files. An ARINC 661 compatible CDS must have a Server capable of creating the GUI hierarchy during initialization, thus avoiding the need to be recompiled if the GUI definition changes.
ARINC 661 was originally created with the expectation of being used on Air Transport aircraft only. Thus, the feature set and overall design are conservative. Today, ARINC 661 is viewed as being useful for Cockpit Displays on Air Transport, Business/Regional and Military aircraft. ARINC 661 development often happens in a certification context (RTCA DO-178B/EuroCAE ED-12B), and this fact has influenced the contents of the standard.
Current status in FlightGear
ARINC 661 also has (very basic) GUI support: http://en.wikipedia.org/wiki/ARINC_661 And the J661 project does provide a WYSIWYG GUI editor, and there are plans to also provide a widget editor, too 
Over the years, we've seen a number of discussions related to ARINC 661 support in FlightGear 
As of 03/2016, FlightGear does not have any dedicated A661 support. However, there's a separate open source project, J661, that can be used to hook up an ARINC 661 emulator to FlightGear. Also, some A661 concepts have inspired related work in FlightGear . Having a dedicated CDS component in FG that runs A661 instruments would allow avionics manufacturers to test-run A661 avionics in FlightGear. 
In theory, it would be possible to parse a A661 DF (XML based) and turn that into the corresponding Canvas properties to recreate/use A661 avionics in FlightGear without requiring J661 or a JRE.
For the time being, using it for prototyping and simulating ARINC 661 instruments will need a fair amount of work, because FlightGear really "isn't there" yet.
- Hooray (Jan 17th, 2013). Re: Canvas custom GUI.
- Hooray (Feb 27th, 2013). Re: how many man-hours it took to develop 2.11.