i just googled a bit about onenote, and i'm not so sure that tomboy matches up. tomboy is just way simpler.. but! you should check out
basKet, 'cause i think that's the closest linux alternative you're going to find right now.
it's in gutsy's universe repository, so it will be simple to install from synaptic. the only shitty thing is that (i assume) you're using gnome.. and basket is a kde application, which means synaptic will have to install a bunch of dependencies (kde libraries, the qt graphic toolkit, etc.). it should work just fine, but it will look different ('cause it's using qt, rather than gtk)..