Posted this on Twitter a week back, but maybe it's of some interest to blog readers; here's how to do simple voice synthesis on the iPhone 3GS (3GS-only, I'm afraid). It's a private API, but hopefully if we file enough Radars they'll make it a public one.
To enable the following code to work you'll need to link the VoiceServices.framework (from the PrivateFrameworks folder of the SDK) in your app.
NSObject *v = [[NSClassFromString(@"VSSpeechSynthesizer") alloc] init]; [v startSpeakingString:@"All your base are belong to us"];
Apologies for my lazy 'NSObject' define above, but you get the idea :-)
With that, you have simple speech synthesis for your application (obviously you cannot include this when you submit to the App Store as it links to a private framework, but you can use it in your internal applications). It requires the 3GS because the Speech stuff just isn't in the firmware for the older devices.
Obligatory video below: