Exported wallet.dat from bitcoin-qt. Then uninstalled bitcoin-qt. Now the wallet is empty! Some months ago, I installed bitcoin-qt on Ubuntu, created a wallet and address, then transferred funds to it from MtGox. I let it sync up for over 10 days, but it never fully synced. So I exported a wallet.dat file, then uninstalled bitcoin-qt.

Today, I installed MultiBit and imported the wallet.dat file. It is empty. It has one address which shows no transactions or balance on blockchain.info. The wallet in bitcoin-qt had several addresses.

What happened? Is there anything I can do to restore the files containing my bitcoin?
I had the same problem. What I had to do is delete the copy of the blockchain and let the newly installed bitcoin qt download and sync from scratch. All was good after that.