I have a wallet file I backed up June 2011, and has been offline for over a year now. When I copied it back into where the files are stored with the official client, it says my balance is 0, with no transactions.
Does it take a long time for my balance & transactions to get updated correctly, or is my wallet considered corrupted, and the coins lost forever?
Any unspent bitcoins against addresses in your old wallet should be retrievable.
The client won't show transactions that haven't been downloaded to your copy of the blockchain yet. There should be an icon in the lower right corner of the Bitcoin-QT client that indicates progress on downloading the blockchain. It can take a few hours (there's around 2.5GB to download, and it downloads them slower than you'd expect because the client verifies the blocks as it goes).
There is no limit to taking the wallet offline. The blockchain, which is shared by all users, contains all the information on who owns the bitcoins. Your private key identifies the ownership.
Now the reason why you may see a balance of 0 is because the official windows client doesn't seem to store the wallet inside the application folder, but in your OS user profile. It may still be reading the wrong wallet.
If you see your old transactions, but a balance of 0 - then you should worry.
I think the problem is that the Bitcoin-Qt client doesn't know that you have changed the wallet, and for performance reasons doesn't rescan the blockchain for transaction relating to the new wallet.
It's easy solved by starting Bitcoin-Qt with the -rescan parameter.
