The latest version of ReBirth - 1.2 - is now in the App Store! Both the iPhone and iPad apps have been updated, and the newest feature of this release is WIST support.
On both devices the WIST button is located with the other menu options.
Tap to start and the iPhone or iPad will start looking for other devices running WIST.
Once the devices find each other* they'll show up in the menu.
When you use WIST, first device that's selected will be the Master - the one that controls playback, tempo, etc. - and the other will be Slave. Once the Master's been selected it needs to allow the Slave's incoming connection, then close the WIST window, and the devices are ready to go!
In addition to ReBirth, you can use WIST in Tabletop and Korg's iElectribe or iMS-20, so grab a device and a friend and sync up and start jamming!
* WIST works over a Bluetooth connection - not on a wifi network - so make sure this is turned on in your settings!