I had the same problem just now after. After reading many threads I figured out how to sort it out. Open itunes:
Open iTunes, plug in the iphoneiPhone/ipadiPad/whatever...
- riteRight click on the iphoneiPhone on the left hand side menu and select reset warningsReset warnings.
- click store>authorise this computerClick Store > Authorise this computer. (I used the same username and password that I used in the app store)
- Then riteRight click the mobile again and select transfer purchases/Transfer purchases.
This worked for me perfectly without causing me much of a hassle. I do hope this was helpful to you.
Cheers LakmalsL