Stig,
1 - yes, dng color profiles consist of transform matrix and additional lookup tables to slightly modif colors after conversion that was performed using color matrix. Matrix in dng (provided by dng4ps2 for example) is disregarded and the one from profile is used
2- workflow is very simple - you have to save color profile in camera raw map. Dng4ps2 is used as is, no matrix change for that camera needed. After opening dng files, you just select embeded matrix or one of color profiles for that camera (if you have more than one). For details, see the thread DNG color profiles - end of color problem and follow the link to Adobe Labs site
3- if you have a dng from Ixus, you won't se profiles for A650 in ACR, so you can't use them
Anyway, as I said, I tuned color profile to give me same colors as Adobe color profile for 400D. Rendering of in-camera jpegs is slightly different, but I was satisfied with Adobe's rendering. You will try and see if you like the result or not. It is also possible to tune individual colors further with Adobe profile editor and save modified version
