According to the Insight book and a 1986 Awake article, C-14 dating is reliable for all dates SINCE the flood (2370 BCE or so).
There is also the matter of that pesky post-flood Egyptian civilization. Not only do many Egyptian stone tools back to 5500 BCE or earlier - but Egypt has a written history going back to 3300 BCE - that's right, a written record starting 1000 years before the wt flood.
So apparently durring the -3130 years between the flood of 2370 BC, and the early stone tools of 5500 BC, Noah got off the arc, some of his family traveled back into time, so that they could provide Egypt with stone tools and the hundreds of thousands of slaves needed to build the Step Pyramid in 2750 BC (also pre-dating the wt flood).
The entire Egyptian civilization is all supposed to be all post-flood mind you.. so there's no radical change in 'atmospheric radiation' to blame here... (Note also that modern archaeologists employ several dating methods, not just C-14. Each dating method has a specific range and accuracy, but by combining techniques they have been able to increase accuracy. They would have certainly noticed if atmospheric changes were causing C-14 to deviate significantly from values deduced using other methods... infact, they have detected such deviations and they adjust for them.)
- Lime