Given the slot constraints at EWR and the fact that UA and CO were both due to take delivery of new 787s in 2012, perhaps CO37 will be upgraded to a B764 or B772 for summer 2012? This would also depend on suitable stands being made available at EDI (see my other posts on the current PCN limitations at EDI and the potential solutions to that issue).
Given the delays to B788 deliveries and therefore the uncertainty about when B764s or B772s might be available to be cascaded to other routes, and given the uncertainty about the availability of suitable stands at EDI, perhaps its not surprising that UA/CO are delaying announcing a second flight from Edinburgh next summer.
(I'm assuming that EDI would not be a priority for shiny new B788s but I could be wrong).