There was a long thread about hammer down on a 1911 on THR, while yes you can do it, and it is relativly safe, its not very smart.
I'd also like to point out, that a 1911 is very safe with out the safety engaged. 2 things must happen for it to go bang, which is not the case for most "new" semi autos. those it only takes 1 thing. Now I don't recomend carrying like that, but it still does not change the fact that it is safer then say a glock.