Maybe, there's something temporarily wrong with your Comcast service on some of those channels. The TV in my bedroom isn't a HDTV, but my channels still work (having DISH Network is likely the reason why everything works for me), and most local networks in my viewing area have made the switch to digital already. I hope your issue gets resolved soon.
Sparky, it's true that the digital transition date got pushed back again (honestly, I wish they'd stop delaying it.
No matter what, there will be few folks that will never be ready), but many networks voluntarily made the digital transition in February). Again, as for jury duty...yuck!