noOnespecial, the questions that you ask are not quite as simple as they have been made out to be thus far.

Will the taxpayer pick up the tab for a United pension default? Perhaps. And that's a HUGE perhaps. The pensions are insured by the PBGC, which is supposed to cover any gaps in pension programs. They charge a premium to ensure that they have funds to cover any and all defaults. http://www.pbgc.gov/about/default.htm
Yes, it's a government agency that insures pensions. Have there been previous defaults? Yes, plenty; if you are old enough, you remember USS, LTV and a few other steel companies defaulting on thier pensions. They were pretty big pension defaults. United and every other company with a pension plan picked up the tab for that with higher premiums.
As for the figures that United is short on pensions, keep in mind that those numbers are based on current pension payouts, not PBGC payouts (which can be SIGNIFICANTLY lower). There is also a factor of the extremely low interest rates, which (on the surface appear to) compound the shortfalls. There is still quite a bit of money that will be turned over to the PBGC; probably more than will be needed to cover any pension obligations. I can envision multiple scenarios where PBGC makes money by taking over United pension plans.

There are a few larger issues that you did not raise.
For instance, are pensions dinosaurs? I say yes; they are being replaced by 401ks. How many companies offer pensions anymore? Is it fair for one company to fund pensions while another does not? That is not a level playing field. If a new company moves into your market, is it unethical to match their lowball prices, even if it generates a loss for you? Should new companies be permitted to offer prices well below their break even cost?

You've opened up a larger can of free trade/fair trade issues. From your posts, it appears that you lean heavily in the Independence Air camp. By no means is Independence Air hands clean on the issues that you raise. This is not a simple matter of a tree falling in the forest; there are many dominos that fall when one falls.