Go Answer, then come back....
"So I discovered on Buffy Forums today that the majority of fans think that Dawn's keyness definitively ended in The Gift, and that any story that involves her still being the key would require a major retcon. I don't really want to argue the case here. What is puzzling to me is that I've always thought the exact opposite. I thought it was obvious she was the key, and that while her keyness was dormant, it was always around to become a plot point again. It's just weird when it turns out that what you thought was a plain fact is the opposite of what others regard as a plain fact. So I want to find out how out of the loop I am on this."
Is Dawn Still the Key?
As far as what Joss thinks (and ergo, what is canon) is impossible to know. He could very well just pull something Key-related out of nowhere to give her a plot point... but, here is my 'what it should be' opinion:
Yes - Dawn remains the Key as that has taken the place of a human soul. This doesn't necessarily mean that she could open any more dimensional gateways. It is possible she had one use and one use only - opening a gate to Glory's world at a very specific point in time under a very specific circumstance. My point is that Dawn still isn't strictly human and can't be because she isn't 'souled'. The Key energy takes the place of a human soul within her.
So, what will happen when she dies? She'll be returned to her natural Key-energy state - whether that means she will have consciousness, or simply dims into purely unintelligent energy, I'll leave to fanon. It wasn't clear to begin with what sort of sentience she did or didn't have prior to the monk's reality warping spell (only what they thought and what was recorded in Watcher HQ in some book - we never received objective confirmation of anything they stated).
I have my own choices for Dawn's development already clear in spanderverse - but that doesn't mean it is so in the main-reality Buffyverse. But I would have to believe that someone with the knowledge of her nature and location would be able to utilize that energy, when it is no longer contained within a human shell (and possibly even though it is).