Build an online wallet service with Bitcoind, can it scale? If not, what can be done?
I hate to be this guy but if you're struggling with this chances are that you'll struggle to secure the private keys. Those private keys are the keys to your empire in a way you don't even want to think about.

Think very seriously about outsourcing this to a company with insurance for it. Or get insurance yourself. The outsourcing also takes the performance hassle away from you.

If you don't, please don't forget to make (really) cold wallets. Just place that cash elsewhere.

Perhaps more directly, there's several bitcoin daemons out there. You could make your own, too, with something like bitcoinJ. I met the guys behind btcd ("BitcoinD in Go") https://github.com/conformal/btcd at Bitcoin2014, their lead coder made a good impression.

I'm afraid there's no shelved answer for this question yet. Perhaps the best advice is "Do it, once you run into trouble you're successful already. Time enough to solve it later.".