Well, my answer is still the same as the last time, although this time I made an example rather that just saying "use a camera sprite with Scroll-To behavior and scale the movement the way you want".
I found a bug in C2 (and will file a bug report) - if you press and hold the middle mouse button, but release it out side your game window, C2 still thinks the mouse button is down. (EDIT: it only happens in FireFox, so it is not a C2 bug - it probably is a FireFox bug.)
C2 can not prevent the mouse from leaving the game window (unless you go full screen). So, the drag camera function can not completely duplicate the way Starcraft does it... I remember reading a thread a few months ago about some proposal for future browser/HTML5 updates to allow that functionality but couldn't find it just now...
as a bonus, I included a 9-patch selection box that I did recently to help someone distinguish between clicks and dragging to select units. I also made a clickable mini map, and added zoom on the main map.