Exactly tomturd. I can't code in Flash (ActionScript) and while I considered utilizing someone else's code for this, you have licensing issues to contend with, especially in a paid product, and you have support issues. What if a user is in a corporate environment with a locked down IE7 and no Flash installed? They'd simply be unable to upload files? That isn't acceptable.
Additionally, many hosts don't support Perl (I think we actually don't support it, for example, though we also don't support APC).
The reason I went with APC is because
1) It doesn't require anything special on the user's side, so it's up to the admin whether or not it can be used
2) If the user has javascript disabled, while other things don't work in IPB, at least the upload can still occur (fails gracefully, at least moreso than the other options)
3) It's easier to "check" for in the script (you can't check if perl is allowed very easily, nor can you check if the user's browser supports flash very easily)
Oh yeah, ETA
--As I understand it, PHP6 will be including APC in the core release, so you won't have to install anything special.