Umm everyone is saying 1, but no. There are 2 (and a third coming or already there).
There is no difference in the hardware, it's all about the software.
The first one (A) should be long gone by now. It had the original software on it.
The second one (B) came out around January/February of 2008. It had the January software upgrade so you get Youtube, Maps, Customizable home screen etc.
The third one (C) which should be here now (or shortly) has the new iPhone software features, like download apps directly on your phone, Outlook Exchange Server Mail, Multiple Calendars...etc.
The price differences you see are from retailers clearing out old stock for the newer versions. you can grab a great deal now since the upgrade is only $10 (depends on where you live, check ipod.com) to go from A/B to C versions. You can save extra money, and if you're like me, you might not even care that much about the new upgrade since third party hacks have worked fine for a long time.
You can see what version they sell by looking at the box. On the bottom where the bar codes are, look for the Model Number. MA*****/A is the first version, MA*****/B is the second one. Presumably, MA*****/C will be the third.
Apple doesn't officially talk about the different versions. They jsut list one model and they sell the newest one on their store. But deep down inside Apple, they consider it different versions. For all the years past, iPod software updates have always been free so no need to give a new model number. But now they're not (since they add new features), so they need new version numbers. So they are different versions, but since the hardware inside didn't change they still call it the same generation. This is fun yes? Look for the version and price you're happy with.