Sure, there may be some remarkable technology in ancient civilizations. My point is this: if the U.S. was wiped out for some reason, there would be remains that lasted for a VERY long time. Or maybe something could completely and utterly destroy all evidence, but if that happened, do you really think the Empire State Building would still be standing, but everything else gone? It doesn't make any sense. The Egyptians may well have had technology we don't know about or don't understand.

As to the dating, check out RichBarna's Nova link.

The argument for the Egyptians building the pyramids: all the evidences suggests there was no one else to do it.

The argument against: we don't understand how they did it.

I don't know how they did it, but the simplest explanation is that they did. I can't completely rule out the possibility of extra-terrestrial life helping them (), but there's no evidence to suggest that. We've all got wild-ass conjectures. I've got my own. But I think you're reaching on this one.