A few days ago, I noticed a sign in a window of a building on Plum Street just north of Fourth indicating that a veterinary clinic will soon be opening. Building Cincinnati (in all likelihood to be announced as 2009's best blog in CityBeat's Best of Cincinnati edition next week) had this story back in January.
It seems to me that this is the type of business geared towards neighborhood residents; I'd think it unlikely people will drive downtown to have their pets cared for. So I view this as a positive sign regarding the number of people living downtown. At some level of population density, there should be some sort of tipping point that will have lots of resident-geared services coming downtown.