We have published a lot of research at Gartner (here is a research note that describes criteria for evaluating innovation from ecosystem partners that links to some of the other research) on the concept of Ecosystems and how the big enterprise application vendors are starting to build them. Phil Wainewright at ZDNet did a good post identifying the constituencies that are trying to get into the Ecosystem game.
We have not seen too many HCM vendors jump on the Ecosystem bandwagon yet, but if some of the early vendors have success, it may spur more movement to align with a specific ecosystem providers. Keep track of which vendors have become ecosystem partners as you make decisions about new HR technology. In addition, dig under the covers to find out if the vendor has just a marketing relationship with the ecosystem provider or if they have really created deep product integration (including standardizing on the ecosystem vendor technology platform/architecture). Not all partners make the same level of commitment to a particular ecosystem.