There has been much discussion about this for as long as I can remember, going back into the 70's. For one reason or another, any plans (or even talk of plans) gets a rapid 'NO WAY' response from various local officals, conservation groups and the resorts themselves.
Technically, an interconnect would take 5-6 lifts, and not be a big deal. Even connecting the three canyons via tunnel/pass would be a pretty straightfoward feat. DV & PCMR already adjoin one another, as do Solitude/Brighton & Alta/Snowbird. Guardsman Pass is not an option however, as the road is unimproved dirt for at least 5 miles of the way, and other parts are on road bed not suitable for heavy traffic or snowplows.
Economically, I would guess that it's a no-brainer, except for DV. All the resorts would see signifcant skier number increases, as people would visit for the curiosity factor alone. DV would hesitate, because they value a low skier density experience and uncrowded day lodges. In fact, I've heard that a DV/PCMR hook up is currently on hold due to the potential of day lodge overcrowding and no other reason.
But here comes the big NO: local back country and wilderness enthusiasts would never let this happen, and I have to agree with some of the arguments. The Wasatch is a unique range. No where else is there a combination of terrain, snow and wilderness so close to a large metropolitan area (SLC is the only city in the US with a federal wilderness area within its boundaries). Its over use is already being noticed. with the local ecosystem being stressed to its limits. Seven resorts in such close confines to the wilderness areas already take up a large portion of the range. The resorts have done a good job of re-tinkering infrastructure to maximize the user experience, without gaining too much new territory. If a solution was available that offered nearly the same acerage as is currently available, I would be for it. To me tunnels are the best solution.