So much has already been said about this cup final, there's not a great deal more that I can add. Jonathan Wilson has written some great pieces (for the Guardian and Sports Illustrated) which discuss the potential benefits of a 3-man United defence - but he naturally concludes that Ferguson won't risk such a dramatic change in strategy, especially not one which requires him to break up his preferred centre-back pairing of Rio and Vidic (United have conceded just four goals in the competition this season! - although, Barcelona have scored 27...).
And Michael Cox has previewed the game excellently, as well as highlighting some important lessons from the previous final of 2009. He points out that one of Ferguson's biggest problems is that, with Darren Fletcher unfit, a midfield pairing of Ryan Giggs and Michael Carrick is nowhere near sturdy enough to deal with Barca's Busquets-Xavi-Iniesta(-Messi) central midfield. He also questions whether Chicharito ought to start for Man United - something which is likely to be Ferguson's most significant tactical decision.
And elsewhere, everyone's favourite Champions League-winning manager Rafael Benitez lent his name to a dazzling double-spread preview in The Times.
So I'll just offer my own humble prediction of how each side will line up, and, consequently, where the game might be won or lost.
Everyone knows more or less how Barcelona are likely to set up. With Abidal unfit, Mascherano will probably come into the defence and push Puyol out to left-back. The consensus on United, however, is that they will start in their usual 4-4-1-1 formation, which they have used for most 'big' games in the second half of the season, with Chicharito up front and Rooney just behind. However, I think this line-up makes more sense, and I think Ferguson will be tempted to use it.
It allows Park, the ultimate harrying 'defensive forward,' to close down Busquets, who, as Sid Lowe has pointed out, is absolutely integral to Barcelona's play. During (inevitable) periods of sustained Barca pressure, Park can drop deeper onto Xavi, allowing Giggs to pull wide and track Alves, while Rooney can drop onto Busquets. This allows Nani, one of United's shining stars this season, to stay permanently higher up the pitch, where, if the ball breaks quickly into the left-channel, he can be one-on-one with Mascherano who he can beat, or at least draw fouls and bookings from. It might also encourage Alves to stay deeper, which would severely hinder the fluency of Barcelona's attacking play. As such, Nani would become the only player without a serious pressing remit, but would be regularly afforded space wide, and would be able to isolate one of Barca's weakest links (as strange as that may sound when talking about Javier Mascherano - he is not a natural centre-back).
From a defensive point of view, of course it still relies on United's back-four remaining solid - retaining shape is absolutely vital, because Barca's players will wait and wait for their chance to capitalise on any players being dragged out of position. David Pleat wrote in the Guardian that Rio ought to be brave in stepping out of defence to track Messi, but Messi, playing as a false-nine, tends to drop into the centre-right position, which would mean Vidic would have more responsibility for him. If Vidic is left too exposed, though, and is constantly dragged out of position, he is liable to get in trouble with the referee.