I personally believe enough data is available to say almost positively that the Great Pyramid was built between ~3500 and 2400 BC. My guess is that it was within 50 years of 2700 BC. This is not based on the Egyptological timeline which is likely very accurate relatively but skewed absolutely, but rather it is based primarily on C14 results. There is some possibility that contamination of samples caused by working and reworking these sites in ancient times has resulted in erroneous results.
I believe we'll have some excellent specimens to date very soon. ;)
There is a small possibility that ancient dating was tied to some astromical or other datable event that will be found as well.