Most external hard drives at Best Buy are even USB3 capable. The newer processors should usher in USB3 and if you are making thousands of units all at once, you have to consider the long term as you don't want to have 10,000 units which are obsolete as you will lose money when people won't buy them for the target price. It seems to be the consensus that USB3 will be the next dominant system to replace the current USB2, and personally I agree. I would not buy an external hard drive without USB3 as with USB3, you will have a much faster connection to your hard drive and I am willing to bet the next gen Macs will have USB3.
One way to go safe for sure is with the OWC drives which support USB1/2/3 and FireWire 800 and/or eSATA all in one external HDD or SSD. The flexibility is great as you do sometimes wind up using a computer with odd technology (I used FireWire 400 not too long ago) and having one hard drive which does it all is convenient.