Yes, BA moved to Amadeus in circumstances of some controversy. They were expected to plump for Sabre (OW member and all that) but didn't. Neither did QF, which moved at about the same time IIRC.

The major airlines will normally list their flights will all the major GDSs, because they'd be at a big disadvantage in any that they didn't list in. The GDSs basically sell their retail products directly to TAs, so that's the usual bunfight in terms of trying to get the best deal for the business (both ways). It's a bit like picking which bank you're going to use to supply your credit card processing equipment. Obviously there are some geographical regions where certain GDSs are stronger and more widely installed than others.