Any external camera is going to be clumsy to carry around in your pocket with the Pyra. Also, my goals for a camera is more of "digital camera replacement", not "phone replacement"...
So - you want the equivalent of a $200+ dedicated digital camera to be crammed into this already packed device (there is no 'left over space').
A quality digital camera needs a few things simply due to the physics of light.
1. Good lens diameter. This is the window the light comes through. Early phones had more or less pin-hole lenses. Newer ones have larger primary lenses that jut out a bit from the back of the phone to have room for the focusing optics. This is so that it is physically possible to gather enough light to make a quality image.
2. Quality optics. Once you have the light - how it is focused/distributed between the lens and the CCD makes a difference. Poor optics will result in vignetting, color and focus aberrations.
3. Enough distance between the focusing optics and the CCD for the light to 'spread' over the surface area of the CCD. The larger the CCD, the more light it can 'see', but the more distance required to allow the light to 'spread' out to the size of the CCD.
4. Megapixels. Stated last because it's actually the least important anymore. Phone lens, optics and internal focal length means that anything over about 3 MegaPixels simply doesn't matter - it's all marketing from there on a 5mm primary lens. Where this does come in to play is in the density of the CCD - which is it's ability to collect and differentiate the path of the light coming through the lenses.
What this all comes down to is that there is a lot of science, art and technology that goes into making a decent digital camera. There is a very good reason why pro photographers still use 50mm <=F1.4 lenses on 15+ Megapixel cameras with >=1" CCD and are willing to pay thousands for a solid rig. The simple physics of light come into play and 'bigger is better' does apply when collecting light.
So, no. You're never going to be able to cram a 'quality' camera into a 'thin' phone or the Pyra.
