Some random thoughts-
1) how does the US walk the line between showing support for peoplelwho are being gunned down in the streets by thugs who stole an elction, while still not taking sides? The last thing the anti government demostators need is and excuse for the mullahs to tar them as American stooges.
Plus, whoever wins, Obama wants to work with. So he dare not offend anyone. (Yeah, I admit, cheap shot snark.)

2) If the mullahs really did bring in Hizbollah thugs, and proof gets out, things will get even nastier.
3) I think all the groups-the Army, the mullahs, Ahmadinejab, Moussavi- feel they are standing on the edge of a cliff. If they back down they lose. If they escalate they may still lose, but risk things falling apart very violently. And all sides really are pretty much alone. No one from outside can come in to mediate or to help. Scary time.