I've carried a number of cameras -- throwaways as well as digitals. Yes, the digitals weigh more, but I love being able to look through the photos as soon as I get back to my room. (Hey, the fast folks have the same obsession with their Garmin-recorded mile paces, so I don't feel too bad!
) I also get far better quality from my Canon Powershot A570 IS and Nikon CoolPix L4 digitals than I ever did with the instamatic-type -- and mine really don't weigh THAT much more.
Truthfully, I have probably had at least 5 digital cameras that I carried during races over the years -- and while I tried to get accustomed to the stripped-down ultra small ones, I find I enjoy my Nikon and Canon the best, despite the weight. If size and weight are an issue for you, the only advice I'd offer is to avoid any zoom -- as the heavier lens and mechanics that enable the zoom feature mean it will weigh somewhat more. I'd also advise getting the largest monitor available, as it seems to make picture-taking easier for the cast members who so kindly will do that for you on race day.
I usually carry mine in a camera case on my belt as I find digging around in my fanny pack a bit awkward. (I'm also terrified I'll lose my license, my cash, or my room key that I carry in my pack.) I tend to run/walk with it in my hands as I go through the parks, then put it away.
For me, seeing the photos after the fact are what make the races memorable. They really "make" the race for me and I'd be devastated if I finished a race photo-less. Not everyone has a similar obsession. As with everything else, I'd "test-drive" a camera during a shorter race before committing to carrying one over a half marathon or longer distance. Similar to a fanny pack, some folks can't tolerate them; others can't live without them. You'll have to see which camp you fall into.
