The protesters know that the PM flew in to Chiang Mai yesterday, but they're not about to pull out of the airport. It's strategically important to them. They have said they'll continue to keep the airport closed until the PM resigns, so it's stalemate.
What happens next at the airport may well be key to whether or not the uprising spreads to the streets nationally. It's interesting, because I don't think the army are gonna get involved if the government asks them to. It looks like a not very well disguised military encouraged coup.