Just to bring this thread up to date, A4 configuration AR-15's are now allowed in the CMP, providing a few things -
a) you have to have the regular A2 carry handle style rear sights with an A2 style handle on the Picatinny rail;
b) you have to have the regular A2 triangular front sight and post
c) you can have either an A1 or A2 style buttstock - no adjustable buttstocks allowed and you can't switch stocks in a match
d) you can have a four-way front Picatinny but it has to be full length and the front sling mount has to be at the front end bottom of this rail.
There's more in this year's rules, but that's the update to 2011. I know this thread is years old but it's what still came up under Google when I looked just this week, so I thought I'd help set things straight for the moment.