On another note, I found this puff piece article about Sarah Palin and her family in the February 2008 issue of Alaska Magazine.
It is unclear when the author interviewed Palin, but the author does mention seeing Bristol at the Palin home. It was the night of a gala ball that the Palin family, including Bristol, was attending. The article mentions that Vogue came to Alaska in December 2007 to photograph Palin, so the article must have been written sometime between the Vogue shoot and the article's February 2008 publication date.
The article mentions Bristol driving to Anchorage from Wasilla to buy a special dress for the ball and spending her gas money in Anchorage to get her legs waxed, to the chagrin of her parents who thought leg waxing was a waste of money. This doesn't jibe with Bristol being pregnant in December or January (when I assume the author visited with the Palins), but it also doesn't jibe with Bristol being home sick with mono.
I've done a bit of research on this, though have not talked directly to the magazine. However, another blogger, who has had a blog promoting the idea of Sarah Palin for V.P for months, mentions the article, encouraging his readers to buy the issue, on January 22nd. So we know that the magazine was out, and available by 1/22/08 at the very latest.
This is a full color, full production glossy magazine. Knowing that printing and distribution schedules on publications like that usually run a month or more, I can see no way that this article was written in January. I am assuming it was written sometime in December, after the Vogue shoot, which was December 8th. This would mean that whether Bristol or Sarah was pregnant, the pregnancy would only have been around 18-20 weeks.
While it's clear the reporter noticed nothing amiss with Bristol, it's also interesting that she describes Sarah as dressed in a "black skirt and silver-sequined sweater," so no sign of pregnancy there either. One of them had to be pregnant.
Yes, it would be nice to figure out what event this was, since we can find absolutely no photograph of Bristol with her family after later summer 2007.