One of my friends has a Carbon-15 in 9mm and it's a very light gun.
There are a few things you can do to get ultra-light builds, short pencil profile barrels (like a 10" or 11" barrel) with standard front sight base and standard handguards, and something like a magpul flip-up rear sight, or a Daniel Defense one.
Check out the thread called "PDW builds" since ALL those are ultra short rifles, a lot of them being AR15's and such.
A company called Plum-Crazy makes a fibreglass reenforced nylon lower receiver (very inexpensive and very light) then you could try for a slick-side upper receiver (AKA no forward assist button or shell deflector) I think that NEA (North Eastern Arms) makes a slick side billet upper, although it has the shell deflector, it's going to be lighter than a standard upper.
DPMS also makes a light upper without FA or deflector (or the door cover) which would be very lightweight, and since there are DPMS dealers here, you should be able to get ahold of one: ht tp://www.dpmsinc.com/store/products/?prod=821&cat=1704 (past link into browser and remove space in http)
Questar at the top of the page carries DPMS stuff, maybe shoot em an EMail or ask them in their sponsor forum if they can get ahold of one for you. Shouldn't be much more than a normal upper to get.