Did Sarah Palin cover up daughter's pregnancy?

Following news reports that Republican Presidential nominee John McCain's newly selected running mate, Sarah Palin, may have exercised bad judgment in taking a nine hour flight while 8 months pregnant and after her water reportedly broke, rumors have been flying that the baby may actually have been born to her own 16-year old daughter.

The unsubstantiated rumors seem to have circulated around reports that her daughter had not attended school for a long period of time after contracting mono. Further, Palin was said not to even be "showing". This is one of the explanations she gave for why Alaska Airlines allowed her to make the nine hour flight back home from Texas. Here's a report (excerpted from the popular blog The Moderate Voice):

[Governor Palin's] oldest girl is rumored to have actually been the one who had the last baby, the one with Down's Syndrome. She was taken out of school the last 4 or 5 months of her mother's pregnancy. On March 5th, 2008, Alaska's Republican Governor, Sarah Palin, announced to the media that she was 7 months pregnant with her 5th child. She is currently 44. Palin's daughter Bristol is 16 and attends an Anchorage high school. Students who have attended class with her report that she has been out of school for months, claiming a prolonged case of mono. Palin does not appear pregnant in any recent photographs. The announcement came as quite a shock to people who had worked closely with her, and have been quoted as saying that she did not appear pregnant whatsoever during the prior 7 months.

Palin has reportedly been accused of abusing her power as Alaska governor to try to get her ex-brother-in-law fired as a state trooper. An investigation is underway. When asked if she was Ready To Be President, Palin responded: "Absolutely. Yup, Yup!"