I've never really been too settled on the identification of the 'we' in Genesis 2 and 3. Who are they? Who are the 'us' that God makes us like? Who is the 'us' that Adam and Eve either became like or were once like?
Many people in my circles (Australian evangelicals) have taught in sermons that 'knowing good and evil' is the concept of usurping God's role. It means to determine good and evil, something that is not our place to do. This is because they take the more traditional translation that Adam and Eve 'have become like us' as opposed to Green's 'were like us'.
One exegetical consequence of this is that the identity of the 'us' is more limited. In 3:22, the 'us' are the 'knowers' of good and evil. If the phrase 'knowing good and evil' is understood to mean determining good and evil, then the 'us' can only include the divine. 'Us' cannot mean the heavenly court. (and, by the way, if you take Doug's take on 3:22, there's no exegetical problem with taking 'us' as the heavenly court anymore)
Thus, you're left with some kind of plurality within Yhwh being mentioned within the first couple of chapters of the Bible. A proto-trinitarian statement. It seems to me that God is not referred to in this way throughout the rest of the Old Testament, despite certain ambiguities later on in Zechariah, Malachi, etc. I personally would find it very strange were it to be the case here.
I can't see any evidence internal to Gen 2-3 that favours a 'determining good and evil' interpretation of that phrase. Can any of you? Things I've missed?