The Pittsburgh Marathon added a 5k to the weekend's races. This article describes the race, and the course doesn't seem too inventive, but it was certainly an afterthought for the organizers. It's admittedly a compromise to the popularity of the race weekend, which is fantastic, because so many people are interested in running the half (though we could use more people willing to step up to the full, there are plenty of spaces there still). With a little more planning next year, a 5k that takes advantage of Pittsburgh's hills for a fun and fast race. Note that I plotted this first course in April 2010, long before the marathon course was changed to Liberty Avenue, which necessitates a shift, perhaps to Smallman Street. I could redraw the courses, but since I already waited 11 months to reveal my prescient genius (along with my ego and sense of sarcasm) , I am just getting these ideas out now for...nobody to read them or take them seriously.
A second option is down Bigelow Boulevard, which will be more scenic, though it will involve a few turns. You do get to finish on Grant Street, which is pretty sweet. I prefer this option.
Start the marathon and 5k races at the same time. If the slowest 5k runners come through in an hour, which at that point would be a trickle, the fastest half marathon runners will just be approaching, and the 5k will have been going on for 1:15 by the time an appreciable crowd comes through for the half.
The big logistical issue for both of these races -- getting runners to the top of the hill.