The debtor can propose to reject its pension obligations in bankruptcy if they are underfunded. The debtor has to make a good faith proposal to the union and negotiate with the union. The court can only allow rejection of the pension contract if, the union refuses to acept the debtor's proposal without good cause, and the balance of equities clearly favors rejection of the pension contract.
This occured in the United Airlines bankruptcy, which has dumped its pension obligations on the PBGC. As stated above the PBGC likely won't fully cover the pensions.
Northwest Airlines also does not have a fully funded pension but is trying to negotiate for a long term payback of its obligations so it doesn't have to reject the pension plans. I doubt it will be successful.
EDIT: forgot to mention that most often contracts to provide retiree health benefits are rejected and the PBGC doesn't cover health insurance obligations.
EDIT again: FAQs on PBGC's obligations: http://www.pueblo.gsa.gov/cic_text/e...on/pension.htm