I think that folks often don't realize how *compatible* 7, 8, 9, and 10 speed components actually are with each other. The cog thickness difference is too small to matter, but the spacers often matter. The derailleurs shouldn't matter as long as their reach is long enough, and as long as the shifters have the right spacing. I'm running 8-speed shifters with a 7-speed freehub with 8-speed spacers as well as an 8 speed freehub with a mix of different cogs, but with 8-speed spacers. (7 and 8 speed spacers are close enough anyway that it shouldn't matter.) The lighter wheel with the tighter cluster is the 7, so it has less dish. The beefier wheel with more dish is the 8 and I've got a wider cluster (and bigger tires) on it. The tandem is coming (Monday!) with 11-30t 8-speed, and I've already got a bunch of extra cogs to play with, so I'm planning to mess with it the same way as I get a feel for it and feel the need to customize.
See
http://www.sheldonbrown.com/k7.html . You can also do fun things like run 8 of 9 on a 7 wheel with 9-speed shifters and spacers!