Russia has used several of their old ICBMs and SLBMs to launch satellites, as have we.* We could do the same, like we did with the Titan II. I don't know how much payload a Peacekeeper could put into LEO, but it'd probably be from 1-2 tons.

However, there is some issue about who would own the boosters. Those missiles belong to the Air Force. If we used them to launch satellites, you'd have a government entity competing with Boeing, LockMart, SpaceX, Orbital Sciences, etc. The market for launch services is pretty small, so this competition would really hurt, especially as it would directly compete with the Pegasus and the new SpaceX Falcon I boosters.

The government could sell the missiles to companies that could then use them for launch services, but how much would they charge for them? The Peacekeepers cost many million dollars each when they were bought, but they've effectively been amortized by now. This could also undercut companies like SpaceX who are trying to sell new boosters. There is a historical precedent: following WW-I, the government sold hundreds of surplus aircraft. This made it very tough on the companies trying to sell new planes (why pay several thousand 1919 dollars for a new plane when you could buy a surplus Jenny for $500?). This hurt aircraft companies for several years until the glut of surplus planes dried up and new technology made the old planes less desireable.

*The Russians adapted their R-7 ICBM as a space booster starting with the launch of Sputnik 1. That booster has been evolved over the years into today's Soyuz booster, a very prolific (some 2000 of different versions have been launched), reliable (95+%), and cost effective boosters anywhere. We evolved the Thor IRBM into the Delta series of boosters, the Atlas ICBM has been used for decades, and the Titan III/IV evolved from the earlier Titan II. For some reason, I don't think we ever used the earlier Titan I ICBM as a space booster, but I don't know why. Russia has continued to use old ICBMs as boosters, partly because they don't have as many concerns about undercutting private industry.