The app, billed as a "a portable channel guide for your iPhone/iPod Touch" has a beautiful interface that blows away the crappy on-screen guide that Comcast gives us.
I stumbled across What's On yesterday on a fluke and got attached to it when I was able to quickly find stuff worth watching on TV that I never would have discovered using the clunky, four-line guide that comes with Comcast cable.
No word yet on why the app was removed but several have recommend the free i.TV (iTunes link) as an alternative.
Update: (from the developer)
Thanks for the email. Apple transitioned our developer account from a single license to multi-license account and in the process managed to fudge up our sales ability. We realized last evening that the app was no longer available after apparently everyone in Apple went home leaving us with no way to resolve it. We have been in touch with Apple and it should be back on the app store. Sorry for any inconvenience