I found this interesting link on the future of the SOPMOD program as envisioned by NSWC Crane. The handout is from the August 2001 Joint Services Small Arms Symposium.

It shows an interesting future for the various components of the M4 and the whole concept of a modular rifle itself. One thing I found interesting was the lack of iron sights on the block II and block III concept rifles presented in the handout. Hopefully that was just an oversight on the artist's part...

www.dtic.mil/ndia/2001smallarms/taylor.pdf

PS - For more mind-numbing bureaucratic fun you can dig through the unclas budget figures for USSOCOM - $350,000 for paintballs in FY2000.
www.dtic.mil/comptroller/fy2002budget/budget_justification/pdfs/procurement/tab15_socom.pdf

When will it all end...I went broke bringing my AR up to the "latest" mod...(sigh)...

The two changes that really caught my eye were the improved magazine and improved bolt. Those seem like changes that all AR-15/M-16 series rifles would benefit from.

I'm curious as to what exactly they changed though - hopefully a little sturdier ejector on the bolt for one.

Edited to add:

Well, this didn't answer all of my questions but its an informative document specifying specific issues with the M4A1 carbine and the SOPMOD kit. Its already gone from the web, so you'll have to catch it while it is still in Google's cache:

www.google.com/search?q=cache:a_tZIq7bqtgC:www.crane.navy.mil/supply/01r0102/01r0102.htm+M4A1+Improved+Bolt&hl=en

See also:
www.google.com/search?q=cache:RnBdDrJsFLsC:www.crane.navy.mil/supply/01r0102/rifflespec.PDF+M4A1+%22Improved+Bolt%22&hl=en&start=2