Well, every year they have to develop a new version of the flu shot for the new virus strains that are circulating that year. The swine flu is just another flu strain, and the swine flu vaccine is made in the same way as the regular flu shot by the regular manufacturers. Of course they haven't tested the vaccine with this year's particular flu strains on millions and millions of people (so as to have a chance of observing the rate of very rare complications), but its the nature of the beast when trying to make a vaccine for a rapidly mutating virus. But they do keep statistics on reactions and complications to the yearly flu shots, and thus base their safety estimates off of those.
I'm not sure if this paper is publicly available, but here is a paper with a table on vaccine adverse events for influenza vaccines. For example, one concern is Guillain-Barré syndrome, which was associated with the 1976 swine flu vaccine. But they have since changed the way flu vaccines are made and there have been other manufacturing improvements. The incidence in recent years was 1-2 per million vaccines, which is not zero but quite low.
The CDC also has pamphlets on risks and everyone's doctors should have an opinion. The current estimates of swine flu deaths in the US aren't that much worse than a regular flu year - they are guessing up to 90,000 for the swine flu where a regular year has 30,000+ flu deaths - but there are some important population differences. Regular flu regularly kills old people, but people in their late 50s and older have some immunity to the current swine flu due to exposure to an earlier swine flu strain. Compared to regular flu, the swine flu has greater than usual lethality to young people and pregnant women, especially those pregnant women farther along. At least, those were the statistics from the swine flu strain circulating this spring/summer (among a population that was heavy in connections to Mexico.) Whether those statistics hold for the traditional fall/winter flu season and for the wider US population, we'll have to wait and see.
Whether or not pregnant women choose to get the vaccine, they are supposed to be on greater than usual lookout for flu symptoms and seek treatment if you come down with something. I don't normally bother with the flu vaccine but that's more laziness than choice, and I think I would get the swine flu one were I preggo. I went out to dinner last week with old college friends and during dessert the guy sitting next to me said he'd been exposed to an active swine flu case and was on tamiflu! I didn't know I was at risk. Oh, well.
ETA: Congrats on the boy steph! What is in the water with all the XY little ones
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