To clarify Bryan's post a bit, the "5.5 generation" tag is not an official designation but simple a slang name we/the discussion forums gave the second version of the 5th generation Video iPods to help differential between the two somewhat different types of 5th generation iPods as they are the same "family/line" but did have some differences (5.5 Gens got better battery life, brighter screens, a music search function, and possibly other things that slip my mind at the moment).
If you try to find them in the Apple support pages for specifications, the "5.5 Gen" ones are referred to as "Fifth Generation iPod (Late 2006)". Specs can be found here...
Fifth Generation iPod (iPod with video) 30 GB, 60 GB (10/12/2005)
http://support.apple.com/specs/ipod/iPodwith_video_30_60GB.html
Fifth Generation iPod (Late 2006)
http://support.apple.com/specs/ipod/iPodFifth_Generation_Late2006.html
Patrick