I don't know whether this has been mentioned, but providing that Sir Alex has decided to go with Tevez in place of Berbatov, as the weekends selection would suggest, I would be tempted to start with Ronaldo through the middle for at least the first 25 mins, or so, possibly longer.
The reason for that is two-fold. Arsenal will almost certainly attempt to start the game in a similar manner to United in the first leg. Tevez and Rooney would provide the necessary cover for O'Shea and Evra when Arsenal's fullbacks inevitably attempt to push on and create the overload.
Ronaldo, starting from a central position, should be able to take advantage of the gaps left by one of the two fullbacks. When Arsenal's attack breaks down, Ronaldo can make an arching run in either direction (towards the left flank, or the right), in preparation for a ball played down the sides. Whether he could then get in on goal, or simply hold up the ball long enough for support to arrive, obviously depends on the circumstances.
Now, you may think that it would be easier for Ronaldo to take advantage of gaps left by a marauding fullback if he actually starts in the wide position, but I would disagree. For one, you don't want make it obvious that that is what you intend to do. Even if the fullback is caught up-field, it's highly unlikely that Ronaldo will be left unmarked. It's also much easier to squeeze him out if he starts near the touchline, not to mention that, from the central position, there is a far greater area of pitch to run in to, both with the ball, as well as without.
Ronaldo can also take advantage of any nerves in the Arsenal backline, particularly in the opening exchanges, and pace is likely to give you the best chance of that. And there is also the potential for him to do what he did against Porto.
