Pitch Effect question

I am attempting to recreate the vocal effect used in this song here…

http://www.youtube.com/watch?v=Y6ljFaKRTrI&feature=related

The way I understand it, the usual method to create that effect would be to overlay the vocal track with a midi of the same song. However, I was thinking, -and forgive the terrible explanation, I have no idea about the technical terms for this - wouldn’t it be possible to make a plugin that could take a sample and emphasize the pitches that are included in the standard musical spectrum (or if not just emphasize them, bend them to the nearest musical pitch), wouldn’t that give a similar effect? (and if it’s at all doable, could someone point me in the direction of a plugin or procedure that does just that? )

Really, thanks in advance, I really hope you guys have some ideas.

That sounds like a Vocoder effect to me.
Very similar to what you were describing in terms of how you use it - you need 2 tracks, the vocal track and a tone track. You then apply the vocoder to the two tracks and it makes a kind of modulation of the two tracks (its distinctly different from other kinds of modulators, and not really a modulator at all, but I can’t think of a more appropriate word…)

You can get a LADSPA Vocoder plug-in that works with Audacity from here: http://www.sirlab.de/linux/descr_vocoder.html

I’ve not used the Vocoder for doing anything as musical as you are looking at, mostly just used it for weird robotic type effects, which it does very well. To get a pleasant musical effect will require a lot more subtlety and it may take a bit of experimentation to get the effect right.