I don't really know what iPhoto's Enhance actually do, so, I guess we could start by finding out exactly what it's doing. I'd wager some mix of auto-levels, sharpen, and color adjustment.
I'd like to, then, do something as similar as possible with the ImageMagick shell tools, or, if need be, with some programming thrown in through a library like RMagick.
I'll be processing batches of digital camera pictures of people.