Thank you all, this is great perspective.
I've looked into taking a class or two where I work, but there's a lot of red tape around admins doing academic stuff. I'll do some more digging and see how that goes.
Regarding letters, I'm definitely going to ask my undergrad advisor for a few reasons. The first is that he has written letters for me before, and evaluations of my work were always to the effect of "this person is brilliant and will make important contributions to the field..." so that's definitely a plus. He was constantly pushing me to pursue a philosophy PhD, and I remember it was a major disappointment when I didn't. I don't know how he'll feel about me picking it back up over a decade later, but I figure it doesn't hurt to ask. He got his PhD from the #1 school for philosophy of religion (at least according to PGR, which I realize is controversial), and he's tenured, so it's would be a major bonus to have him in my corner. FWIW, my grades in undergrad were pretty good in philosophy, but the overall GPA was dragged down because I went to a SLAC and was required to take a bunch of math and science classes that I bombed.
The other letters will probably be one of the art historians and either one of my colleagues or a philosophy prof if I can take a class or two here. We'll see what happens, I'm not in a rush to apply next year, though I suppose I could if I got on top of it right now.
One last question - does anybody here have anecdotal information about the state of women in the field now? When I first considered PhD programs 10 years ago, things were pretty abysmal and it was a contributing factor in why I decided not to go. Things seem a bit better now, especially since the APA is getting on board, but I haven't found a lot of positive information about what it's like for female grad students in the field.