Well, who's to say you would have scored the penalty? What difference would that penalty have made? Nothing, because Arshavin scored anyway and you got your goal advantage.
Arsenal did have the better of the play overall, they looked more threatening, but in all honesty, they didn't create that many clear cut chances either. Maybe Diaby should've done better with a scuffed shot, maybe on another day Foster wouldn't make an amazing save to prevent van Persie scoring, maybe on another day van Persie's over-weighted free kick would've sailed way over instead of hitting the bar (if it was any lower it would've been saved by Foster anyway).
Rooney went down because there was contact, what do you expect him to do? At least there was contact, unlike with Eduardo, and Eboué in this match. Those are what you call ridiculous dives, people. Two in two matches? I think Arsene needs to have a word with his team.
It's unlucky that Diaby can't head a ball, but seeing as Almunia left it to him, we got gifted a goal. However, Rooney was lurking behind Diaby, so if he made no contact, I'm pretty sure Rooney would've grabbed a second.
United were below par today, but even then, overall, United shaded the attempts, while Arsenal shaded the possession in the end. Arsenal did get more bookings, but to be honest I think the ref got the majority right. Fletcher would've got a yellow card if the penalty was given, but what would you rather have Arsenal fans? Fletcher on a yellow card and a chance to score, or a goal? I know which I'd go for every time.
Also, on another day, when Berbatov came on, we'd have rattled another 2-3 goals in during the last 5-10 minutes. We cut you open many times when you had to go for it in stoppage time, we really should've finished it then. United were still lucky, but you need luck to win sometimes.