You've got the best reply by far. I think you hit on a really good point - the operating system. With a Pi, I'm running standard linux. With Android/iOS I'm using their SDK. It could be good or bad. It's somethign to learn.
I'm comparing it against a RasperryPi because I'm aware of the Ardruino and the Pi and they are made for the hacking community. But the cellphones aren't and given the fact that we're returing these devices every two years there is a _HUGE_ market of very capable hardware out there.
I have several projects - from OBD2 projects to water control (on/off with a flow meter). I can do the water control except for configuring the device flow amounts. So add on some way, and with WIFI I can rig up some rudimentary web interface and write a file on the Pi. The ODB2 project needs GPS and WIFI and a display. I've been shopping around and I keep having to buy more and more addons and I realize, I have 2 iPhones and and Android (2.3.5). So why am I just not using them? Oh right, no hardware pins...
Others have suggested something called an IOIO, which looks to be exactly what I'm looking for.
My question isn't flamebait at all. If you're threatened by outdated cellphones being superior to hardware you like, then that's your problem. The irony here is that a six year old device with multiples of computing power and storage is going for $0.99/$1.25 on eBay (iphone 1g/Aria). The only thing it's missing are the IO pins and the open OS... The irony is that IO pins are cheap.
So let me as you this, wouldn't it be far better for you to get the IOIO and an Aria for your projects? You wouldn't be limited by your hardware. You could give a GUI, even a basic status one.
(Admittedly the IOIO is a bit expensive. I'd figure it'd run about $5, but there's a considerable amount of implementation to bring in USB OnTheGo, and all that to work around the manufacturers not implementing host mode in phones from 6 years ago...)