Iphone can be very annoyingWhile trying to update the Facebook AppStore application on my Iphone, I got this "very informative" message: The application "Facebook" cannot be opened! I've googled a little bit and found a few solutions: 

1. Reboot Iphone
2. chmod 777 application folder
3. Remove application from Iphone and reinstall it using checkboxes under list of applications on my iphone
4. Delete application from Itunes and download it again

 but non of them worked. 4th solution was a hint! ;) When you delete an application from Itunes make sure that it's removed from the computer (hard disk) but before deleting an application, right click on the application and choose Get Info and see where an application in stored on disk.

 This was the issue for me since I have one application that I shouldn't delete (purchased under different conditions). So I've not moved it to trash/recyle bin when Itunes kindly asked. I've moved the application from folder where Itunes is storing apps, did sync, double click the application, Itunes imported it, did re-sync and voila that was it ;)