We've written about KeynotePro before. They make very nice themes for Keynote that you'll be happy to use for your presentation.
This week, they've updated the "Ohm" theme for Keynote '08. It's been completely rebuilt to use shapes and placeholders native to Keynote '08, contains up to 58 master slides (the Pro version) and two palettes.
The Pro version includes 16:9 and 16:10 aspect-optimized versions, and the new OM '08 EP Edition is optimized for the iPod and iPhone. They look just as great on those tiny displays as they do when connected to a projector via video out.
Both versions require Keynote 4.0 or higher, and the Gill Sans and Helvetica Neue font faces. A single license will cost you $24.95US, though discounts are available for bundle orders and those who purchased earlier versions of the theme.