I'm trying to develop a bitcoin service, but I can't seem to figure out how to hook into the "bitcoins received" event in the bitcoind client. Is this even possible? Do I just have to poll the client every so often?
Yes, you have to poll.
Usually, with most websites, you have a customer order to something that gives you a hint to start polling.
In the future, we'll have some 'push' gadgetry that will do what you want, but that does not exist yet.