[h=2]Chainrings/Cranks[/h] [h=4]Old Chainrings, New Chains[/h]There is a lot of confusion about the compatibility of narrow 9- and 10-speed chains with older cranksets. Shimano says you should replace the inner chainring(s) with specially designated 9- or 10-speed ones, but then they're all too eager to sell you stuff, whether you need it or not. These chainrings have the teeth slightly farther to the right than the older chainrings to work a little better with the narrower chains. There is no difference whatever in the crank spiders.
The manufacturers also are concerned about clueless users. The worst-case scenario is that you will be riding along with the bike in its highest gear (large front, small rear) and then for some bizarre reason shift down in front before downshifting in the back. (There is no shift pattern in which it is reasonable to shift in this sequence.) [Not with a 9- or 10-speed cassette, to be sure -- John Allen] If you do shift this way, there's a small chance that the chain might "skate" over the edges of the teeth for maybe half a turn.
In practice this "problem" almost never materializes. Many, many cyclists are using 9- and 10-speed chains with older cranksets and having no problems whatever.
[h=4]New Chainrings, Old Chains[/h]Going the other direction, using wider chains with chainrings intended for narrower chains is not generally a major problem if there's only a one- or two- generation difference. The only problem you might run into is that the chain will be more liable to rub on the inside of the bigger chainrings in the small/small crossover gears, gears you shouldn't be using in any case.