How to store wallets online? Something like blockchain.info appears to keep full wallets online. Do they run a standard bitcoind on thousands of servers (one per person)? Or are the somehow managing to keep individual wallets separated?
Blockchain.info probably doesn't use the standard bitcoind client, but something written just for their use, which works like bitcoind, but with a large list of encrypted wallets instead of just one.

Since bitcoind just isn't made to handle thousands of wallets, it would be cumbersome to try to have a web wallet like blockchain.info use bitcoind. I don't think you'd need one server per person, but you'd need one instance of bitcoind running (whether on-demand or always running) for each wallet.

I'd expect that nobody would use the standard client for this, but would fork (or create from scratch) their own version. Maybe at some point, someone will release such software as open source software, but there's not really a need to have many different servers like that (unlike, say, mining).