I didnt see this posted but found it over at Slickdeals.net..
If you are currently a Digital Rebel (300D), Rebel XT (350D), Rebel XTi (400D), or Rebel XSi (450D) owner, register your camera at Canon USA's website (http://www.usa.canon.com).
Then register your Canon EOS 40D or 5D from now until July 19th, 2008. Fill out the form, send in your 40D/5D UPC and proof of purchase from an authorized dealer.
Canon will send you a $50 rebate for upgrading.
The 40D/5D purchase must be made between May 18th and July 19th and you must have already registered your Digital Rebel, XT, XTi, or XSi prior to July 18th to be eligible for this offer.
Here's the PDF:
Pretty sweet deal if you're planning an upgrade (which seems to be the topic of most discussion here at POTN )