There's a simple hack for enabling it on OSX (it worked by default in some iOS 4.2 betas but got disabled). In fact, I think this works for anything running a CUPS server (i.e. Linux, BSD, etc.), so doing this in an embedded box should be pretty straightforward. Windows is probably trickier.
Tangent:
Of course I say that, yet I've seen plenty of Linux-based consumer grade routers fuck up printer sharing despite using CUPS. I suspect most of the usual suspects (router makers) won't get in there before Apple. (plus they'll completely fail at marketing that aspect if they do)
I often wonder if there would be a sufficient market for a "better Timp Capsule", i.e. a well-done, pimped-out NAS/Wifi AP/router/printserver, specifically targeted at iPad/Smartphone users (but which works fine for other computers as well). The user experience of all the devices out there (apart from Apple's) is absolute shite, and Apple's are kind of limited.