Smart Sign

Summary
Smart Sign is a special app that shows product specs and is installed on the "Smart Sign" iPads inside Apple Stores. This app can’t be exited with the home button. Supposedly, there is a gesture to quit apps on "Smart Sign" iPads which is shared on a need to know basis among employees, and these gesture does not return you to the home screen but returns you to AppleConnect. However, this has never been verified.

Technical Investigation
The app itself is not what disables the home button; it is system-wide and enforced by SpringBoard. The key SBStoreDemoAppLock in com.apple.springboard.plist enables this functionality. A disassembly of Springboard.app/Springboard reveals that there is no special gesture built in to Springboard to emulate the home button outside of multitasking gestures (if they are enabled on an iPad that was connected to Xcode). The only way to quit an app when this key is enabled is holding HOME and POWER buttons until the device reboots. As in Apple Stores the POWER button cannot be pressed, you should hold down the upper-right corner of the iPad's frame.

Apple sets the key in com.apple.springboard.plist by using a mobile configuration profile installed on the device. The profile is set to prohibit removal of itself. If you want to try it on your device, you can find here a similar version that works on any device and that is not locked.