I had always heard (via the A.net and FT rumor mill) that the CO-EK alliance is purely a marriage of convenience (much like CO-VS). It has long been rumored that CO would much prefer to send its own aircraft nonstop to DXB out of EWR, but lacks the widebodies to do so. Additionally, EK doesn't seem to be too keen on joining an alliance. They're funded well enough that they can just spend their way towards clobbering all the major international players (BA, SQ, LH, etc...).

I think CO would jump at the chance to have a partnership with a M.E. airline with excellent service, and a good route-network, that wouldn't necessarily be looking to stab CO in the back at its first convenience.

If CO partners up with Etihad, I would imagine that the EK arrangement would suffer a quick death...much like the CO-VS tie-up would unravel the moment CO gets LHR landing rights.