It depends on the state. Alaska, Arizona, Calfornia, Iowa, Minnesota, Montana, North Carolina, North Dakota, Oregon, Washington, and Wisconsin are non-recourse ( http://www.tnr.com/blog/the-stash/when-banks-cant-go-after-defaulters ) Although I seem to recall some fine print about California and refinancing, and I thought some states limited non-recourse loans to primary residence, although this article doesn't mention that, so definitely check with someone about your individual state.
The reason the average upside-down homeowner doesn't have to pay taxes on it is that you don't have to "when your total debts are more than the fair market value of your total assets" (but get a good tax preparer). This is not the same thing, and you don't necessarily have to file bankruptcy.:
http://www.irs.gov/newsroom/article/0,,id=174034,00.html
(Don't know why I can't get above link to work, but copy & paste and I think it will work.)
As to your credit rating, I couldn't really speak to that, but it's a good question.