Desk Pet TankBot - The adorable TankBot is a tiny motorized toy car that you can control with your smartphone (compatible with both Android and iOS). Apart from using your phone to drive it around, the TankBot has a free-roaming mode as well as an "automatic" mode that uses its infrared sensors to move around objects. It charges via a retractable USB port and is currently available for $25.
Orbotix Sphero - This adorable ball can be controlled with your iPhone or Android device. As funny as it may appear, it's a pretty interesting concept. Sphero uses bluetooth to connect to your phone and you use the accelerometer to control the ball's movements. At $129.99, it's a bit pricey. However it looks like Orbotix plans on creating more apps and games in the future that can be played with the Sphero. You can even draw a route using your touchscreen that the Sphero ball will follow.
Griffin HELO TC Helicopter - Accessory maker Griffin has plans to release this mini iOS controlled helicopter this holiday season (compatible with iPhone, iPad and iPod Touch). It uses a "Flight Deck Module" that attaches to your iPhone's headphone port, using sound signals from the headphone port to the "Flight Deck Module" to send IR commands to the Helo TC. Not using WiFi or bluetooth keeps the price down to a reasonable $49. One impressive feature: you can record a flight plan with the iPhone controller app and have your Helo TC replay the same route you recorded.
Parrot AR.Drone - The AR.Drone from Parrot originally started the smartphone control toy scene, providing a futuristic interface (using the phones accelerometers) and making smart use of multi-touch screens. Originally the AR.Drone was only compatible with the iPhone, however Parrot recently support for Android. At $300, this flying gadget is the most expensive toy of the bunch. It does, however, have the most fully featured controller app, allowing you to even use your smartphone's screen to view through one of the two cameras attached to the helicopter. Your smartphone also uses WiFi to communicate with the AR.Drone, so it doesn't need an extra module.