Planning to build own cellphone based sleeptracker project

I'm using Sleeptracker, but I need something to get better data that it provides, and have more flexible work modes.
There are many interesting projects with custom devices based on micro-controllers. I used to solder small electronic devices in past, but now I'm maybe too lazy and just looking for some pragmatic device, that looks and works good. It's fun to make design of own device but there are many ready small computing platforms with suitable features.
I'm thinking about cellphone, or maybe smart-phone, with built in accelerometers and available free software development tools for developers. It needs to have accelerometers and API to get acceleration data from it.

That kind of device has nice features:

• it's programmable so you can put any features you think about
• compact size, good for wrist attachment (ok maybe little too big, but make some trade off)
• built in battery with quite long operating time and possibility of charging
• possibility to use other input devices to get data like camera to get light level, or microphone to catch noises
• builtin gprs or wifi to send data to server
• of course its still cellphone

The biggest problem may be lack of API to needed devices. New devices are more open than in those in past, but some vendors still don't expose access to internal services.