Speaking as someone who once left his iPhone on a park bench, the iKeep from Avanquest seems like a pretty good idea (fortunately it was a school day and the park was empty, so the iPhone was – miraculously – still there when I went back for it).
It's a fairly straightforward little device, consisting of little more than a chunky plastic belt clip with an extendable cable curled up inside it. The cable has a dock connector on the end, so you can connect it to your iPhone or iPod to prevent you from dropping it, or from putting it down – on a park bench or anywhere else – and forgetting to pick it up again.
The cable can extend to a length of two feet, so you can leave it clipped to your belt and pull the iPhone up to your ear to answer a call or to tap away on some apps. When you're finished using the iPhone you just give the cable a little tug and it will retract back into the belt clip.
It's not, admittedly, the most elegant iPhone accessory we've ever come across, but it does the trick and stops me from worrying about losing the iPhone all the time. However, It's not quite the anti-theft device that the manufacturers suggest – the cable and the dock connector simply aren't sturdy enough for that – so it's still worth keeping your iPhone hidden out of sight whenever possible. We've seen cases with metal belt-clips that are a bit more rugged, but you still have to unhook the iPhone in order to use it.
Just remember that the cable connector is the older 30-pin type, rather than the new 8-pin Lightning connector, which means that you can use it with most iPods and iPhones apart, of course, from the iPhone 5.
The cable could definitely be a bit sturdier, but the iKeep is still a handy device that is well worth the price for absent-minded people like myself who constantly worry about losing their iPhone. It'll probably be handy for young kids who have an expensive iPod Touch as well.