I've noticed that several Ripple gateways now exist, most notably Bitstamp. However, the only documentation I can find on how to create an automated gateway seems to imply that you must have OpenCoin's rippled program.
Can anyone build a Ripple Gateway, or is this ability limited to OpenCoin's inner circle?
The documentation does assume you will run a local server. But anyone can build a gateway, with or without the server. To operate a gateway, you really only need to be able to process transactions just like a client does. When a transaction happens you need to know. When you need to perform a transaction, you need to perform it.
Probably the easiest way to do it is to implement the gateway's interface to the Ripple network using JavaScript and node.js -- this lets you use ripple-lib which is a supported, open source interface to the Ripple network.
If you do decide to run your own server, nothing really needs to change. Just point your gateway interface to your local server instead of the public server pool.
Whatever you do, please don't send your secrets to the public server pool. If you use ripple-lib, make sure local signing is on. If you use anything else, use ripple-lib's rsign.js program to sign transactions for you.
If you are not connected to a trusted rippld server then you must make sure all your transactions are signed with your account secret key before submitted them to a public or untrusted rippld server for propagation on the network.
