I have a preban Glock.  If I attach a stock to it does it become a SBR?

Yep.

Somebody read that thread that the CCU carbine conversion units came up in.  Go back and reread the [i]entire[/i] thread.

Yes, it will, unless you also add a barrel >16" prior.  However, there are additional issues on whether or not you can add a [i]folding[/i] stock at all.   Since your Glock was not in "evil" configuration prior to the AWB, the ATF does not consider it grandfathered.  Therefore, even if manufactured before the ban, it is not considered by them to be "preban".  Thus, adding a folding stock would put you over the "evil" features count and have an illegally configured SAW.

Larry at CCF used to sell these, until he got the following letter from the ATF explaining their position:

[url]http://www.ccfa.com/images/ATF%20Glock%20Folding%20stock%20response%20letter.pdf[/url]

And yes, I know ATF letters are only their opinion, what matters is a court of law, yadda yadda.   However, registering a gun with the NFA branch that the ATF considers to be in an illegal configuration probably isn't the brightest idea regardless.   Hopefully in 9.5 months this won't be an issue any longer...

Rocko