Many users, who are testing the Mac App Store have been hit with the message We could not complete your App Store request. An unknown error occurred (100).
According to TUAW.com The fix for this seems to be simple.
The release of the Mac App Store has provoked yet another set of iTunes Terms and Conditions for us all to agree with (seriously, Apple, I think we can all agree we’ve surely agreed enough by now?), but the App Store seems to be flakey about reporting this to you. Quitting the Store and reloading it appears to be enough to bring it to life; you’ll know when you get it right because you’ll be walked through the usual T&Cs process. Other users are reporting success with restarting iTunes instead, or restarting their entire Mac, so that’s also an option if the first approach doesn’t work for you.