There is a tutorial posted at ftp://dude:sweet@ftp.sonicfoundry.com/Sample%20Projects/Transporter/ that would allow you to do something like this.
I believe to get the effect you are talking about you would need to motion blur and speed up the background layer while isolating the forground layer's featured subject. For the street it may be hard to do unless you shoot a background plate first and also one with the subject walking and do some sort of garbage matte around the subject. Problem is VV does not do garbage mattes yet, but you could try doing something close by cropping the image.

The jail cell concept should work fairly close to how the tutorial is set up, except you would have a shot with the subject and a light moving to cast the shadows and than another one with the subject and no shadows. Possible also you need one with no subject or shadows.

Green/Blue screen would work 'easier' if that is an option for the street scene. Check out the Spice Girls video for "2 become 1" for some very nice effects like you mention.

Hope that helps.

PS - ReVision just put out a new plug-in called "Re:Fill" that might help with this. But it is not available for VV. URI is http://www.revisionfx.com/rfil.htm be sure to check out the "Gallery" link because they have an example called "Creation of a character-less background plate". Pretty cool plug-in.

Conceptually (as in, not necessarily how to do it in vegas, but in general), what is most often done is:

1. Shoot clean plate (only background, w/people etc. BUT not subject)
2. Shoot subject in same location in front of bluescreen
- while subject COULD be shot in the studio, the same location is necessary to match lighting and it's easier to match camera angle and focal settings if you simply use the same camera in the same LOCKED-DOWN position.

3. In post, speed up clean plate to desired speed
4. extract subject from bluescreen shot
5. composite clean plate and bluescreen extraction

Notes:
-Camera MUST be locked-down for both shots (unless you have motion-control gear). Otherwise, the two shots will not match, and even an amazing amount of motion tracking may not be able to realign them (due to perspective differences, etc.)
-Remember to shoot the clean plate for long enough to speed it up a good amount to get a real 'time-lapse' effect AND still have enough footage to composite with the realtime subject shot.

Don't have any idea how well it works, but the Icarus software I was turned onto in this forum is supposed to allow you to remove people or other moving objects from a video. In theory, if the scene was simple enough that it worked well in practice, might provide a path to doing this effect without a blue/green screen shoot.

Alas, the software is no longer avail for download, so if it's of interest, ask and someone with online storage might help out.

A much more elegant solution to this effect doesn't require any chroma keying whatsoever. Shoot your actor in his environment and direct him to move very slowly, quarter time perhaps. Also perform all camera movements ultra slowly. Then in the editor, boost the speed of the clip to return the subject and camera movements to natural speed (400% or whatever) and all other ambient movement in the shot will be on caffeine.

This technique requires some practice to keep the camera motions steady, but it's quite cool. Try doing the opposite with fast movements and then slow down the subject.

As always, lets see what you end up with. Post links to your movies in the forum.

The only problem with this is that it doesn't provide a perceived "natural" movement of the subject. As the actor is only moving slower than everybody else, when everything is sped up, the actor's movements have the potential to appear chunky or just plain unnatural. Remember, humans can notice unnatural movements innately.

Thanks for all the replies guys. I wanted to get some ideas having just seen the film (Requiem for a Dream - a good film and a must see for us video editors, but quite disturbing subject matter and depictions.) I have a production coming up in a month or so but it does not require this effect. Still, I may do a test myself for future reference. I'm wondering if just locking down the camera as suggested and shooting a backgound and then foreground with subject, them compositing the two shots played back at different speeds will do it. Today L. Disney posted something similar he shot of a freeway here in Southern California on the DMNVegas forum and it was great.

Re: Icarus - I went to download it right after they pulled it from the site. I'd like to get a copy if possible (though don't want to violate any copyrights/licenses.) Any ideas?

My guess, although Disney wouldn't come clean, was that he placed the same video, taken with a tripod on two tracks, then used maybe a cookie cutter to reveal the upper diagonal portion of the first video and hide the upper diagonal portion of the lower. Then he applied different effects to both, thus making it look like cars were going in the right direction on one side of the freeway while cars on the other side were all backing up, etc.

Remember, humans can notice unnatural movements innately.

Too true. But personally, I would prefer subtle movement anomalies from the method I described to less than perfect blue screening. It's a difference between admiring what someone did in the editor versus what an actor and camera crew does in real time.

Blue screening would not be a viable solutin for a lot of these effects as the actor is moving between the slomo extras. A lot of clever effects can be achieved using multiple overcranked cameras and 3D composites or even still cameras fired in sequence as is done for bullet time.

I could be wrong here but a lot of this stuff is really in the province of guys with very deep wallets, well at least for the next 12 months.