The following are some notes on getting an Ethereum Wallet running on OSX to talk to an previously running geth instance.
If you start your experimenting with Ethereum on a Mac you'll most likely begin by downloading the Ethereum Wallet. This application will install geth locally and automatically start an instance of it and know how to communicate with it automatically. In addition there is a menu option to switch between running geth against the public chain as well as the –testnet chain.
At some point if you are developing an application or more of a command-line person you'll start running geth manually from the command line to keep your chain up to date or to rebuild it. At this point you may want to know how to start the Ethereum wallet to connect with an existing geth session rather than complain when you start the wallet and it notices a previous geth instance and then refuses to start.
Here are the details for OSX. The idea is similar for other operating systems. In short you want to pass the geth.ipc filename and location to the Wallet so it can communicate with the existing geth session.
$ /Applications/Ethereum\ Wallet.app/Contents/MacOS/Ethereum\ Wallet --rpc $HOME/Library/Ethereum/testnet/geth.ipc
If you are running geth against the public network you'll use