Just downloaded the new OS 6 onto my Blackberry Curve 3G (9300) and couldnt find my App World. So downloaded it again through the internet and even again through the USB and still cant find it !
Try downloading app world from Blackberry.com As I recall my last update went sideways and it vanished also. Then I was redirected to an update patch, which also failed. Finally went to bb.com and all works fine now.
Posted from my CrackBerry at wapforums.crackberry.com
Certain AW versions did go poof on the desktop, while a look in the Application List on the phone showed it installed. Best advice is above, download the latest and it should reappear.
Don't give up on OS6 so quickly. Give yourself a couple days to learn it, and see if the benefits outweigh the negatives. For me, tabbed browsing and unlimited downloads by themselves make it invaluable.
I had the disappearing App World problem when I upgraded to OS6. It was in the app list but had no icon. Battery pulls and reinstallation did not restore it, and was not hidden nor in a folder somewhere. I finally fixed it by uninstalling and reinstalling.