As of Feb the 14th, it is possible to insert a strip in between two other strips:
http://projects.blender.org/scm/viewvc.php?view=rev&root=bf-blender&revision=54553
The VSE will simply make room for the new strip by sliding existing ones!
This feature was announced by Ian last year on the TOS blog but I never got it to work.
I’ve just tested it with the latest trunk and it works both with image and with movie strips.
I’m speechless!
As described in the log, to get it to work select strip, press at+T, and then G twice. Then moving the strip in between other strips will insert it in place.
Another VSE milestone, thanx devs! 