I had the same question a few months ago. According to MPI, the answer is "yes" with two caveats: (1) Skymiles redemption on UA (and vice-versa) is to be the same inventory allocated to Mileage Plus "general member" redemption, although I'm not so sure that UA favors Premier and above over general in terms of redemption--we need guidance here from more knowledgable posters; and (2) you must deal with Delta agents in redeeming your Skymiles on UA (and vice versa) and it just seems to me that, at least to some degree, one is at an inherent disadvantage when a DL agent is trying to navigate the UA system in terms of searching for an available FF seat, etc. On the upside, I've redeemed Skymiles on UA (once so far, for biz class SFO-JFK) without any problem at all. (Of course, just for fun, I should have called UA as well to see if I could have gotten the same FF seat on that same flight!)