I remember hearing someone complain about this earlier, and after playing Survival a couple minutes ago, TNT actually does have a timed explosion. After hitting it, it will bounce, and smoke will start coming out of it, then explode 2 or 3 seconds later.
I didn't check to see if arrows set it off though.
I seriously wish notch had posted the 0.27 update info to avoid people posted the exact same info 10 times already, I don't even think some people even noticed there was an update and keep posting saying that people are posting false information, when it wasn't in 0.26.
I didn't check to see if arrows set it off though.
Edit: they don't.