I would speculate that the camera, and the Canon software, have been performing some kind of auto vignetting correction which has up to now concealed from you the true optical behaviour of the lens.
The lens may even have been subject to some engineering compromises, perhaps allowing some vignetting (which is easily corrected for with image processing), as a trade-off against some other constraint.
ACR, including the older version you are using with CS3, includes lens vignetting correction controls. This is done subjectively by eye, to your preference.
Later versions of ACR (the version for Photoshop, not IIRC the one for Elements) and also of Lightroom, add the option to use a lens correction profile which accurately reverses both vignetting and lens distortion in an objective, programmed way.
In due course when you decide to move to newer software, this "profile" feature will make the whole issue of lens aberrations, remarkably, just go away. By then a readymade profile supporting your particular camera/lens will likely be available too - which it is not, yet, so far as I know.
