(warning: broken english ahead)
I've build a lens profile that can be used with the dngs saved on this camera by chdk. I made it with Adobe Lens Profile Creator and the resulting profile is compatible with Camera Raw and Lightroom.
The procedure is quite simple and I could describe it here as suggested by waterwingz but in fact I've just followed the instructions that came with Adobe's software. Although simple, it is extremely time consuming: 9 shots for each of the 10 zoom steps, all perfectly aligned and evenly illuminated. It took me around 4 hours to shoot all 90 dngs plus 1 hour to prepare the files and around 2 hours of processing time on a 2011 Mac Book Pro (i5). I thought of building it for my A640 too but I don't think it's going to happen!
The attached profile contains all the necessary data to correct geometry, CA and vignetting. For CA, you have to enable it in the "Color" tab of the "Lens Corrections" panel.
A word of caution: at the tele end, more correction is done on the left side of the image; maybe a sign that my camera has a decentered lens. YMMV.
Installation:
For OSX users, place the file inside
/Users/(User Name)/Library/Application Support/Adobe/CameraRaw/LensProfiles/1.0/
For Windows 7 or Vista users, place the file inside
C:\Users\(User Name)\AppData\Roaming\Adobe\CameraRaw\LensProfiles\1.0
I compared the camera's jpegs with the corrected dngs and we not only get better correction using profiles but also a wider fov, maybe to the tune of a 25mm efl, horizontally.
Please do not hesitate to contact me if you have questions.