As the adage goes, all that glitters is not gold, and in the same manner, just because it looks like an iPod doesn't mean its actually is one. Imitation is supposed to be the highest form of flattery, but i doubt either you (if you buy one thinking you're lucking out on a good deal) or apple will be flattered by these knockoff media players.
Apple has released a warning for resellers, to be on the lookout for these knockoff, which the manufacturers have gone to great extents to make look like the authentic Apple products. They have also released guidelines for resellers to be able to identify these imitation products. Specifically targeted are the iPod Nano and iPod Shuffle lines.
Things to look out for to make sure you have a genuine Apple product, the on/off switch of the iPod Nano should have a repeat button & a battery light indicator, no dock connector and non standard headphone jacks and of course the interfaces are not like the iPod interface. Ipod gear has some photos of the knockoff iPod products and distinguishing marks to look out for.