I wasn't tempted by Natal when I was a Wii-only user. Hell, I'm not tempted by Move at this point either.
In reality this is only due to the fact that no good games have been shown for either solution yet. Even if/when that does happen, Natal would probably not tempt me as that would require buying a 360. Move could tempt me given the most awesomest PS3-Exclusive option. If its what I think it will be with Wii/PS3 multis, I'll just buy the Wii version.