I am in agreement with a post at DrugMonkey's regarding the No Cost Extension on grants. A one year extension to the time allotted to spend the grant money from your original budget is a welcomed window of time to tie things up. The benefits of the added time are obvious, especially with NSF grants that typically run only three years, the fourth year stretches that dollar a bit further. And if you've been smart about how you spent along the way, it all works out fairly well.
But the idea of an active project has different consequences and connotations at NSF than NIH. Whereas NIH does not limit the number of awards a PI can have at one time, that's a little more of a touchy subject at NSF. I've heard from multiple POs that there's a blurred line around the two core grant line where the funding of a third becomes questionable. Obviously there are a variety of factors to weigh here and one's status on the grant and time remaining are certainly big.
So where does the NCE fit in? Say one is early into one grant and has a second about to go into an NCE. Is that extra year considered a "current" grant, potentially excluding the PI from additional funding, or does it not matter because NSF has already spent all the money they are going to on that grant? Have people found that having NSF grants in NCE has hurt their chances with funding recently, or is the bar so high this hasn't been much of an issue?