I am not totally sure, but if I had to guess I'd say it's firmware. The EXIF data for the RAW and color JPEG read a Paramter 1 setting and the B/W shows being processed in the B/W parameter.

This started after the 2.0.2 firmware upgrade last year. I had the Tamaron lens about a month before the upgrade and it never did this until after the upgrade.

Canon did tell me that some EF-S lens users were getting both color and B/W images when they use a #25 red filter.

I don't mind the extra image, but it's becoming an annoyance. It takes twice the card space and the buffer is cut in half. A 4GB 80x that normally holds 465 JPEG + RAW is now 230

More reasons why I think it's firmware ... the same lens on a 5D and an XTi - no problem.