Bush has now confirmed that attacks have indeed started.
But initial reports suggest that it may have been a "target of military opportunity". There are, apparaently, significant levels of surveillence going on in Baghdad and the US/UK have the option to respond to opportunities VERY fast. This, it appears, is what happened.
The suggestion (unconfirmed) is that an opportunity may have arisen to hit part of the Iraqi regime hard - just as dawn broke in fact. That will have surprised everybody. Not the massive bombardment suggested, and not a night attack.
Speculation is that maybe they got a chance to try to take out either Saddam himself or another significant part of the leadership, and if that actually was successful (and no one is saying yet if this was such an attempt and if it was, whether it worked) then maybe the war could be over before it really gets started.
That really would be the "shock and awe" the US promised!
Anyway, so far, no massive bombardment, but there certianly have been some VERY loud bangs - BBC were live from Baghdad as it happened.