I'm not sure if it is still going on, but if you buy an arcade, you're eligible for a refurbished hard drive to be sent by MS for....20-30 bucks I think? That's as cheap as they'll get and you shouldn't have any problems getting a refurb directly from MS.
The holiday bundles are already out and about in stores. That doesn't mean retailers won't start packing things in themselves, but the arcade comes with Sega Tennis and the others come with...I think Lego Indiana Jones and Kung Fu Panda (but don't hold me to that). Point being is that those bundles are already available.
As far as waiting goes, everyone keeps talking about the Jasper chip, which is supposed to make the system more stable and lower power consumption. But no one knows when those will be available, and most likely they won't be in proper circulation until after the holidays. Additionally, the bundles available now aren't Jasper chips - they are leftover Falcon reserves.
In all reality, it seems that the Falcons are pretty stable, so you shouldn't have a problem getting one of those. So really it doesn't matter if you wait or not - you might miss out on a bundle from a retailer, but I wouldn't know anything about that.