You would have to use pywallet to extract the private keys, then import those private keys to blockchain.info. I do not believe that blockchain.info has the capability to read/store an entire wallet file.
^^This is bad advice.^^
You most certainly can upload your wallet directly to blockchain.info and create a new blockchain.info wallet with it. If you already have a blockchain.info wallet that you want to keep, you can either have two of them, or you can send the bitcoins from one wallet to the other, and abandon the empty one.
Here's the link to create a blockchain.info wallet from a Bitcoin-Qt wallet:
https://blockchain.info/wallet/import-wallet