It seems really clear cut to me. If you get moved by a bullrush you take AOO. Its that simple. Mobility applies, since you are moving, and taking AOO from that move. Personally, I don't give a damn about what the SRD says, for as long as I've been here on the forums it seems to be wrong a lot more than my trusty PH. My book says its provoke AOO, and until I see some statement that says otherwise, I'm going to leave it that way. That's a rant I know, but we change so many things around here due to Sage advice and errata that I think we don't need to start changing something until WOTC actually says to change it.
Whether grappling should automatically provoke AOO or not is another issue altogether. Let's face it, you can't sunder people's weapons that easily, the world is not made of 5 foot squares, and you can't rationalize every combat action in DND except by using the rules we were given. The rules say that movement except the 5-foot STEP provoke AOO (except of course the stated exceptions). There is no 5-foot STEP here, so its an AOO.
And all those tactics that were brought up about bullrush aren't silly, there are excellent strategies and teamwork combat. For instance a rogue darts in on a dragon, tumbling in to avoid AOO and sneak attacks him. The fighter next to him, bull rushes him away (carries him away is a better image) to avoid the dragon's full fury (full attack). Its a great image of a friend saving another friend, taking a claw in the back while doing it (by moving to bullrush someone, you still take an AOO from the dragon in this case).
Or another example, a friend who reacts to danger first pulls his friend with him into the frey. The friend picks up speed and then launches himself at the target.
But anyway, I'm about to launch a thread about that stuff. That's not what we're talking about.