As far as I know Yukon isn't going to be an OS, I thought it is just one part of a new server OS (which is itself called Blackcomb)?
Don't they codename their future products after towns and cities? Windows 95 was Chicago, Windows 98 was Memphis, Windows XP was Whistler (never heard of a codename for Windows 2000, everyone was just calling it NT5)
I thought the next Windows release is still called Longhorn for now, but I have no idea what it will be released as (the Windows 95 name came as a major surprise, remember)