K.K.
the most important (and usually surprising) thing to know is that crunches and sit-ups are an absolute no-no. they can actually make the situation worse. it is true that most cases of diastasis recti (that's the medical term for it) can only be fully repaired by surgery (it's a laproscopic procedure, fairly non-invasive, but still takes like a month to recover). here are some good exercised to do which can help reduce the gap and strengthen the muscles, lessening the pressure on your back and the likelihood of a hernia:
http://www.maternalfitness.com/articles/baks-basics.pdf
I've also heard several people recommend Mummy Tummy by Julie Tupler, though I can't speak for it personally.
isn't motherhood swell?! ;)