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A lot of homeowners may be reluctant to walk away from their home because they believe it is wrong to walk away from any debt, they are afraid of legal consequences, they don't want a foreclosure on their credit, and well, it is "their home". A lot of people right now are underwater in their mortgage and are just chasing lost equity. If you have negative equity in your home, it is not your home anymore, it belongs to the bank. And you may even be paying way above market rates. If you owe way more on your home than it is worth and you are struggling to pay your bills every month, you may want to seriously consider just walking away. But remember it is always good to consult with a lawyer before doing anything.
A lot of times mortgage lenders may not even have the right to come after you (or it may be very hard for them to do so), if you have money in a safe spot (like a 401(k)), creditors cannot get to it if you walk away from the debt, and yes a foreclosure will hurt your credit score, but if you are already in a bad financial situation then your credit score has more than likely already suffered.
On the other hand, if you are only a small amount underwater, it is probably a better idea if you just stay put and try and fix things.