In Mac OS X, I have a website that I host from my Mac.  Typically, all
HTML belongs in ~/user/sites/
I want to be able to see, via http, data that is in ~/username/movies/
How do I do this without coping the data do the "sites" folder?  

When I goto www.mycomputer.com/~username/movies/ I should have access
to this info.

consider using iTools, this way you are just wasting time. You need a
static internet connection first of all. Try using Abyss Web Server
X1, a personal web server software.
http://www.versiontracker.com/dyn/moreinfo/macosx/21361

This is done fairly easily in Mac OS X.  First open up the Terminal,
you'll need to set up permissions for the Movies folder and then
create a symbolic link (an alias that Unix understands). Copy and
paste the following commands, one at a time in to the terminal.  Be
sure to press return after each one if it doesn't execute on its own. 
Make sure there is no existing folder in your home "Sites" folder
called "Movies"

sudo chmod -R 775 ~/Movies

ln -s ~/Movies ~/Sites/Movies

That's it, the links you posted should work perfectly.  Let me know if
you have any trouble.