Those that say "it depends on which state" are correct. It varies from state to state. As for the link Sandman gives, I believe that at the top of that page, there is some sort of disclaimer about how "actual field conditions vary" blah blah blah. This introduction is probably born out of the fact that there are many "dire sounding" states, where, .... truth be told ...... it just goes on anyhow, and no one's ever cared (barring your being a nuisance and snooping around obvious historic monuments, etc...).
The reason for the wide degree of "actual reality" is that a list like the link sandman gives, is born/assembled in the following way: someone .......... oh-so-many-years-ago, simply sends out a letter to all 50 states (especially if written rules they could sleuth out were "iffy" or "unclear" on the matter) asking "Is metal detecting allowed?". Often times some desk-bound legal bureaucrat, who fields such a question, would send back the answer "no", for example. But oddly, in some of those "no" states (on that list), detecting had never been a problem in those state's parks. Doh! So you can see it was a clear case of "no one cares, till you ask" type thing

Perhaps whomever fielded the question morphed something else to apply to the "pressing question" like cultural heritage verbage, etc... (when in fact, perhaps the question never crossed anyones mind before, or rank-&-file out in the actual parks, never cared before).
Here in CA, for instance, there are some where you won't be bothered. However, I don't doubt that if I asked enough kiosk clerks, or wrote enough letters to Sacramento, that I might indeed find myself a "no". Just go look for that boyscout ring your dad lost when he was a kid. And avoid obvious historic monuments, crowds, lookie-lou busy-bodies, etc.....