Something of note: right now, it seems you can only source ATMegas out of China (though I have only checked thru-hole DIPs) - here in the USA, for example, Digikey and Mouser won't have any DIP ATMegas available until sometime in April or May (if I am reading their backorder stuff correctly) - but if you go on Ebay, you can find everyone and his brother selling them out of China (also check ic2ic.com - same thing; tons in China).
I would seriously consider going with an embedded ATMega instead of a full Arduino - you are likely going to have the boards etched, stuffed, and shipped from China anyhow (from a cost standpoint), so an ATMega with programming headers (to use an FTDI cable, for instance - you may also want to include pads or headers for an ISP programmer, too - in case a new bootloader is needed). It will probably be easier sourcing a big supply of ATMegas (well, at least from China) than a big supply of the Arduinos.