has anyone ever seen those apple ipod commercials where it's a person dancing but they are entirely black and the background is like green or something. how would you do that in adobe premiere?

There would be no way to do that purely in Premiere without shooting it a certain way. The best way would be light only the background and expose for the background to create a silhouette effect. You could also shoot it against a greenscreen and turn the subject black later. The painful way would be rotoscoping each frame in After Effects, but no special lighting or greenscreen would be needed.

Could you just shoot the people against a white background then adjust levels to completely blackout the figures then ajust the white screen to whatever color you'd like?

If you watch that commercial closely you'll notice the figures are just about completely blacked out- but there's a short moment where the sneakers or shoes of one of the dancers becomes somewhat noticeable and not completely blacked out- this leads me to believe they were shot really underexposed against a bright background then perhaps completely blocked out in a composting program.
In FCP I'm sure this is possible.

In Photoshop, it's easy to do that with channels...just pick whichever channel has the most contrast, intensify it, save it as a new layer, replace some colors... and voila! Just like the iPod adds.

I bet there's a way to do something similar with Premiere or FCP.

I'd shoot it chromakey, key out the background and drop chroma and luminace until the subject was black. Could be done in P-Pro or FCP easily.

Well I'm just 3 years late for this but if anybody has the answer, how exactly are they doing it for the ipod ads? Are they shooting green screens? over/under exposing the subject and background? Are they doing the rotoscoping by hand (maybe to add the white ipod)?

I'm curious because I have a client who asked me to get that look for a short sequence he wanted to shoot where there will be bodies hanged by the neck. This will later be projected in a theatrical play. He wants them silouhetted with a solid color background. And I'm wondering if the easier approach would be to do the effect in camera or in post.