Thanks for the suggestions - I love this forum!
It doesn't have to be people or mammal's only, anything that moves in the vicinity of a particular direction. I'll look into the linear photo diode array.
It's a little lazy but I'm willing to compromise on accuracy for a quick and relatively easy solution - it's for a quick camera prototype to go in a public space - I want to create the effect of a camera looking at people - so precision is not so necessary.
I'd be keen to know how you detect moving edges with the IR - if your willing to share? I'd rather not have a PC but again, I'm willing to compromise.