The Japanese PLEN robot - that used an Android smartphone as its remote - is great, but at $3,000 it's a little too expensive for the casual hobbyist. Instead, how about using some more affordable LEGO Mindstorms kit; that's what the Enea Android team have done, taking advantage of the Mindstorms' integrated Bluetooth to remotely control it from an HTC Hero.
Movement is handled by physically tilting the phone, and the user can switch between guiding more than one robot. It's not quite as straightforward as pairing the Mindstorms 'bot with the Hero, however, since the Android 1.5 OS the smartphone is running doesn't actually support the Bluetooth serial profile (SPP). Instead, the Hero gets a WiFi link to a nearby laptop, and the laptop bridges the connection via its own Bluetooth adapter.
Still, Android 2.1 supports SPP, and so we can only hope an update to the app is in the works. There's plenty of cheap LEGO Mindstorms kit on eBay, after all.