I wouldn't think NASCAR would be a training ground for F1, not that there couldn't be anyone there who could compete. Seems like they would have to at least go to Indy cars or F3 or something like that.
He might have trouble getting his FIA Superlicense...but then again Bernie would probably pull some strings to help him get it if he saw a chance to get him over into F1. After denying Sebastian Loeb (who is more qualified than Shrub) his SL just a few days ago, it would be a tough pill to swallow if they gave one to Busch.
USF1 needs to drum up interest from fans and sponsors, and they may need to select at least one driver based more on name recognition than on actual skill behind the wheel. Even if Busch has tons of potential, he's untested and an unknown quantity in an open-wheel car. If they sign him, it would be a leap...but a move like this may be viewed as a necessity from the money/marketing side of things within the team. Busch could attract massive attention from American fans, so it follows that he could also attract big sponsor $$.
Of course, it cuts the other way, too. USF1 is a startup team, and Busch has a great thing going in NASCAR. How much money can USF1 offer the untested Busch? How much guaranteed? How long will they be around? How competitive can they be with the big boys at Ferrari, McLaren, Brawn, Red Bull, Toyota, etc? How will he adapt to a new kind of driving in a new car, on unfamiliar tracks, in changing weather conditions, even if the car is great? If the car isn't great, which it likely won't be for at least quite a while, how will KB adjust to busting his *** and being a backmarker or riding in the middle of the pack? These are questions Busch would have to get really comfortable with before making the jump. Of course, he might want to have a bigger view than "Oh, I'd try it for 3 years or so and then come back to NASCAR", too.
Jonathan Summerton is probably the best qualified and most experienced American open-wheel driver out there for an F1 seat. Extremely talented kid and a rising star, imho. His problem for the team is that most American fans don't know him, because he's been off in Europe getting good at racing F1-style. If they were already established from a marketing and sponsorhip standpoint, this wouldn't be such a problem.
Let's face it, Americans know one open wheel race, period. The Indy 500. They don't know what F1 is, because moonshiners from North Carolina don't race in it.
...and that's why PW is throwing KB's name around for a seat. Whether he's serious or not, whether NASCAR would let him even sniff around an F1 test while under contract, and whether the FIA would let him have a license all remain to be seen.