The question of being able to safely carry "hammer-down" on a semi-automatic pistol depends upon what model of pistol you are carrying...
The original 1911, the 1911A1, and the Colt series 70 (and all clones) have a small (read as very fragile) notch on the hammer that will keep the hammer from resting on the firing pin - it holds it about 1/16th of an inch off of the firing pin. If you use that notch, it is safer than actually letting the hammer all the way down, but either way there could
be a problem. With the hammer resting directly on the firing pin, any energy striking the hammer will be directly transferred to the firing pin. That was the reason for that small notch. The problem is that "fragile" notch will easily break if the pistol is dropped and lands on the hammer. The hammer could then stike the firing pin and, though unlikely, cause the weapon to discharge. I say "unlikely" because the hammer would not have as much inertial force as it would if the hammer were being released from the fully cocked position. The inertial force of the hammer moving quickly and striking the firing pin is what causes the weapon to discharge. Now... There is always a possibility of the pistol hitting "just right" on the hammer to cause the notch to break and the pistol to discharge - but like I said, it would have to land "just right."
Liability issues caused Colt to address the so called "problem" and the Series 80 models were introduced. This model incorporated a small lever that was activated by a pull of the trigger. This lever, in turn, pushed upward toward the slide, and pushed up on a firing pin block. When the firing pin block was pushed up by the lever, the firing pin was free to move forward (once struck by the hammer) and strike the primer of the cartridge in the chamber. I do have a Series 80 and found the mechanism to be a "pain" in that it just complicates stripping the pistol. In looking at the mechanism while removed from the pistol, I decided that I would no longer put up with the hassles of it. I simply did not "re-install" it when I reassembled the pistol. I installed all of the same parts that existed in the Series 70, and left out all Series 80 parts. What I found was that it was like performing a "trigger job" on the pistol. And it makes sense... Less moving parts equals less friction equals smoother trigger. I later worked the pistol over with different springs and a Videki adjustable trigger. It now has a trigger of which Col. Jeff Cooper would have been proud. As he described the perfect trigger, he stated that it "sounds like a glass rod breaking." I didn't understand that statement until I finished the trigger job on my 1911... Once I tried it though, I knew what he meant.
Outside of 1911 pistols, almost every other semi-automatic pistol on the market today is going to incorporate some "drop safety" of some sort. Beretta, S&W, Glock, Springfield XD Series, Sig, H&K, Taurus... They all do. One I can't specifically address is the BRNO made CZ75 and all of its clones. The CZ75 pistol is a great design from a reliability standpoint and has been copied by many, including the Sphinx which is marketed by Sabre Defence here in Tennessee: http://www.sabredefence.com/commercial.php?focus=sphinx
These are some high priced Swiss made pistols, but I wouldn't complain about having to carry one!
Hope this helps!!