You can't just switch uppers on an AR. There's too much going on there. You need a new bolt, you need a new mag well or a mag well block on your lower, you likely need a new spring and buffer. People always give other people the impression that "all you have to do is switch the uppers." Supposedly, you can't even do that with a .300 AAC BLK, because you need new springs.
That being said, I think a AR 9mm carbine would be awesome, just with a dedicated upper and lower.
My first choice is from JP Rifles:
http://www.jprifles.com/1.2.13_GMR.php It just came out, seems very awesome. It'd be so cool have a JP rifle. I LOVE that it take Glock mags.
Second choice is from Rock River Arms:
http://rockriverarms.com/index.cfm?fuseaction=category.display&category_id=238 My knock against it is that it takes uzi mags, but seems like a really worked-out rifle. I just trust Rock River Arms as a company. They may not have all the furniture on their rifles that I want, but that's pretty easily remedied.
My third choice is based on prejudice, since I have had no experience with Lonewolf in the AR world, but I like the idea of an AR carbine that accepts Glock mags:
http://www.lonewolfdist.com/Products.aspx?CAT=3682
If you read any literature from any of these companies, they will tell you that you cannot just swap uppers.
Finally, I like the idea of the Cx4. It's light, it's ambi, and it uses common pistol mags. I just really like my AR's and prefer Glock mags to Beretta mags. I'm much more likely to buy a Glock over a Beretta. Just my $0.02.