Update: The tail is down to a little nub; the froglet has been moved to a damp terrarium, and is now adhering happily to the side like a proper treefrog. Unless barking treefrogs have made a move north that I don't know about, it's Hyla versicolor, red tail or no red tail. It's kind of hard to call a critter a "gray" treefrog when it's bright, almost metallic, green! I'll try to get a picture of the newly transformed little guy up later.