Lurkette, my wife and I have a conversation very similar to your concerns about once a week. Basically it starts out with something along the lines of "remember when we could just put Max down somewhere and he'd be there when we got back?"
Infants are much different than toddlers who are much different than "kids" (for lack of a better term). For the first 8 months or so, we could put Max on a blanket in front of the window and go make ourselves a sandwich or whatever. When he was 6 weeks old, my wife would put him in a bouncy chair facing away from the TV so she could watch the NCAA basketball tournament.
First he learned to crawl, then pull himself up and now he's running (we do very little walking in our house). My point is that the changes are gradual. Honestly, we were never worried about mobility so much as sleep, but even that's gotten much better than before. You also learn to cope however you need to. If my wife (since she's the one who stays home with him) needs to make herself a sandwich (or go to the bathroom), she'll usually just waits until he occupied with something for 5 minutes or just gives him something to play with so that she stays in his line of site.
All kids are going to do stupid, dangerous things. When I was 3, my passion was riding my Big Wheel down the driveway, which was about 150m long and a steep hill and into an intersection. Coincidentally, one of my friends posted a blog entry about something he used to do as well today:
http://jackalope_ranch.blogspot.com/
I guess my underlying point here in sharing these stories is that if this is the kind of stuff you're worried about, I think you're overthinking it. While it's obviously one of the biggest decisions you'll ever make and will change your life, the small stuff isn't really important. What's important is whether or not you have the love to spare to raise a child.