macophile provided good info. here are even more details...
3rd gen (dockable) iPod:
1) introduced a year ago in sizes 10, 15 and 30 GB. 10 and 15 are "thinner" and 30 is "fatter." 15 and 30 came with dock, remote and case.
2) updated in sept. to be 10, 20 and 40 GB. 10 is the same. 20 is "thinner" and 40 is "fatter." this 20 is thinner than than 2nd gen. 20 GB. 20 and 40 came with dock, remote and case.
3) updated in jan. this year to be 15, 20 and 40 GB, as it stands now. 15 is the one from #1 above, except it no longer includes dock, remote or case.
"thinner" iPods are the same dimension as the current 15. "fatter" iPods are the same dimension as the current 40. weight may vary but very slightly.