The effect sounds exactly like what Imogen Heap uses, which is synth based.

and

Maybe, looks like the signal is split in two from the mic to where ever it goes.

Signal's defrinitely split in the Imogene Heap vid. Couple things I noticed: the Ableton Live session running on the Macbook and the effects box to the right of the mixer. I think it's a tech 21 but I can't remember exactly which box. There's definitely some prerecorded tracks going on which I think she's triggering with the keyboard. Some sort of vocoder.

DAMN...the VID is already gone. What was it?

If it's like Hide and Seek...read the last paragraph here:
Harmonizer

BTW...I really want that electix looper that Imogen uses in "Just for Now". Those things are fun.

Ok...I couldn't go to bed without knowing what the hell you are talking about...So i did a search on Stewart and found him on Myspace.

Are you talking about the vocals at the beginning of the song Bring Me Down from his Myspace site or could it be the song Slow Me Down by Emmy Rossum which isn't on his page but one of his friends? I'm thinking the latter because the images in the YouTube vid looks like Emmy.

The first sounds to me like simply multi tracked vocals. The second sounds like sampled "Doo" being played back.

OK. I'm pretty sure I've found it. It is indeed Emmy. It's weird, the video won't play from heer but I found it on YouTube. Oh well, this should work.

[[url=http://[/URL]="http://www.youtube…"]Emmy Rossum - Slow Me Down[/]="http://www.youtube…"]Emmy Rossum - Slow Me Down[/]

I think the "Doo"s are sampler but at the end of those lines they aren't doos. It could still be samples but there is a possiblity that those end parts might be just multiple vocal tracks. For the vocals, I think they are multi-tracked however, there are a couple parts that do sound like a harmonizer. However there is a lot of effects so it could be just her harmonizing with herself on multiple tracks.

I think this is more Enya sounding than Imogen Heap. Very nice either way.

...I see myself getting sucked into a female vocalist vortex...again.

There's a live video

which has a guy with a couple midi keyboards triggering her voice, which is what I think is going on. Sampled voices put into ableton live or something similar. But yes this is definitly more Enya than Imogene.

http://perezhilton.com/2007-08-02-listen-to-this-every-once-in-a-while theres a high quality sample of the song in stereo