[alatar]

I assume that we mean that Boromir repents after trying to take the Ring from Frodo. It would seem to me that he would really feel the need to redeem himself - probably why he sells himself in defense of the other hobbits.

So he could have done like he did in the Books (gone in search of Merry and Pippin), or disregarding Aragorn's command, gone with Aragorn and Sam in search of Frodo. If he chooses the first path and survives the Uruk encounter (assume that he is wounded), then he would be a bit slower but a bit hotter in regards to the orcs that took the two hobbits. Possibly he then replaces Aragorn as one of the Three Hunters as Aragorn sees that it's been divinely decreed that Boromir look after the two so that Aragorn can go in pursuit of Sam and Frodo.

Whether Boromir is wounded or not, he still could go after Merry and Pippin or be sent by Aragorn to warn Minas Tirith that the wraiths are now on wings and will attack Gondor soon. By the time Boromir gets to Minas Tirith, possibly Aragorn would have shown himself to Sauron via the Palantir, and the Battle of the Pelennor Fields would ensue. Gandalf and Pippin might be a little more welcome in Denethor's hall, and Pippin still may serve the Steward in payment of Boromir's help (though not death). Boromir and Faramir could go to the bridge to hold off the attack, yet be pushed back as this time Sauron's not just trying to get his Nine across. Boromir could then redeem himself by dying so that his brother is able to retreat.

Denethor could then freak out, blame everything on Gandalf and we could continue on as the books.

Just some thoughts.