New Canon 400D background Subtraction?

1. I was wondering if the the novel dust removal system may be used to take a photo of a background with a tripod then have the object or person enter the scene and take the photo with same background + object again.

Then subtract in the software and voila -perfect extraction of the object from the background? My reading of the software based delta dust removal description in the press release makes me wonder if in fact, this feature could be used this way. Of course it is quite possible that the subtraction is limited to multiple small areas dust size areas only, or some other limitation I have not figured on.

2. Buit what about this subtraction business in general? (with any photo pair)??

In fact why is there no such Extraction feature in Photoshop?? Logically this ought to work with any photo where the camera is fixed and and a background alone and background plus object photo is taken.

Dan