I had posted on here for info about this and no one really seemed to know. Emailed Canon, and this was there reply:
INQUIRY: I have purchased the EOS Digital Rebel and I am going to get my compactflash card. My question is, how fast does the camera write to the card. There are several speeds of cards. I don't want to spend the extra money on the fastest card if the camera only writes at maybe 12x. Thanks in advance for your help.
Thank you for contacting Canon Technical Support.
The only feature on our cameras that will utilize the high speed rated Compact Flash (CF) cards is the Continuous Shooting feature. Unless this one feature is of extreme importance to you, you will honestly be wasting your money on a high speed rated CF card. The CF cards that come packaged with our cameras do not have a speed rating. I recommend that you purchase a CF card with the memory capacity in mind, not the speed rating.
I wish they would have told me what the normal write speed was. Seems like this is imposible to find out.

