I’ve been asked about the video a couple of times, so hope you don’t mind me cutting and pasting:

Software:
I’m using Sony Vegas Movie Studio Platinum (SVMS)
http://www.sonycreativesoftware.com/moviestudiope

which is pretty good.

However this, as with most budget video editors, is very limited in its masking ability so, once I had manipulated/resized the video clip of the overlaid faces, I then had to export a lot of data, make masks in Fireworks and import them back into SVMS.
Then I manipulate size and angles of the masks in the video editor as the clip which is being masked moves. At a rough guess, I’d say 100 to 150 masks were used.
SVMS has a pretty good colour manipulator which, although not perfect, allows some degree of colour matching. Unfortunately, I discovered this facility half way through making the video!!

More expensive, professional editors provide inbuilt mask editors with vector tools similar to some of those you find in Fireworks or similar graphics editors and some provide automated tracking along a timeline. This could possibly save a lot of time manipulating and moving clips and their respective masks but I still think a lot of manual tweaking would be necessary.

In some areas, you can get away with overlaying one or two static images of faces and moving them about but overlaying video clips makes such a difference.

About 1 day into the project (with only 4 or 5 seconds of the video done) I realised that without the right tools or experience, this was going to take ages!
But I was still learning the techniques and reckon I could speed up the process a little.