Ronaldinho played his usual position, which I consider to be on the left of a three.
Inzaghi and Ibrahimovic started as a two, but with one usually moving to the right when they lost the ball.
Boateng played somewhere between right wing and as the right of a midfield three.
It was pretty flexible, and that may have contributed to the lack of pattern in Milan's play.
All they had was long diagonals to Ronaldinho in the hope he could control it, find space and lay it on a plate for Inzaghi/Ibrahimovic.
I thought it was Ronaldinho that wasn't disciplined enough to play as a left winger, and thus acted more like a left sided forward, like he usually plays. But what made me believe it was a 4-4-2, was that Boateng was mainly playing on the right, plus Zlatan and Inzaghi were too central for either of them to be a right sided forward, and Seedorf was playing quite close to Pirlo.
It was a mess out there today, they were so disorganized that they made it hard for people to agree on what kind of formation they deployed.
Whatever it was, it was pretty ineffective, there was little movement off the ball, there was no protection for Pirlo whatsoever, and the midfield was very static, so it ended up in them passing it around for a while then crossing it up to Zlatan in the hope that he could conjure something out of nothing.