According to the Daily Mirror and the Daily Mail, Arsenal could be switching to three at the back.
According to John Cross in the Mirror and Sami Mokbel in the Daily Mail, Arsene Wenger is considering making a major change from a back four and a 4-3-3 variant formation to a back three. The Daily Mail report says that Wenger wanted to make the change before Arsenal's trip to Schalke following a fairly abject defensive performance against Manchester United, but decided against it based on the lack of time there would be to prepare.
Playing a back three would allow Wenger to get all of Laurent Koscielny, Per Mertesacker and Thomas Vermaelen into the side, and would also give license to Bacary Sagna and Kieran Gibbs, as wing backs, to join into the attack with less fear about leaving space behind on the counter attack. And while playing three at the back has gotten a bad rap because of Roberto Mancini's fiddling about with it at Manchester City, a switch to three at the back was crucial to their comeback against Tottenham Hotspur on the weekend; Tottenham couldn't deal with Maicon's overlaps.
None of the articles make clear whether Arsenal would play 3-5-2 or 3-4-3, which leads one to suspect that Wenger hasn't fully decided about what to do in the attacking phase. Playing 3-5-2 would allow Arsenal to play another striker--Theo Walcott or Lukas Podolski, per chance?--while playing 3-4-3 would mean Santi Cazorla would have to be accommodated in the wide areas.
Both articles, though, indicate that Wenger is possibly making this decision based on defensive struggles. With that in mind, it could take time to implement; positioning in a back three is very different to a back four, and as Manchester City have shown this season, if a team isn't drilled in a back three system it can be disastrous. It took Wigan approximately a couple of months to get used to playing three at the back, and while Arsenal are a better side with more intelligent players than Wigan, such a change would be a major shift. It'd be pretty exciting, though, and tactical flexibility is always a good thing.