Hi Steve,
According to...
http://sheldonbrown.com/k7.html...hubs marked "8-speed", "9-speed" or "10-speed" will work with any number of sprockets! (Add a 4.5 mm spacer before installing a 7-speed cassette on an 8-, 9-, or 10-speed hub., and the included 1-mm spacer before installing a 10-speed cassettes on an 8- or 9- speed hub.)
The advice to use an "included 1mm spacer" would seem to indicate the 10-sp cassette is narrower, but I do not have personal experience to verify.
With regards to Dura-Ace,
http://sheldonbrown.com/dura-ace.html states,
The 10-speed cassettes will fit on any Shimano 8- or 9-speed Freehub with no problems.
At the front, it seems generally agreed a 10-speed chain will work on a crank and rings with 9-speed spacing, though some critics indicate it works less well with front-indexed shifters due to a difference in spacing between the rings. Can't verify that, either, but something to keep in mind.
Hope this helps!
Best,
Dan.
