Canon has introduced their Semi-Annual Step-Up Program that is exclusively targeted toward Canon PowerShot owners. The upgrade applies to Canon EOS DSLRs, specifically the Rebel XS, XSi and T1i, and the 50D and 5D Mark II.
PowerShot users stepping up to the Rebel XSi, XS or T1i are eligible for a $50 rebate. For the PowerShot users looking at the 50D or 5D Mark II, you get a $100 rebate.
The devil is in the details though. This is a mail-in rebate. You must have registered your Canon PowerShot camera prior to July 20, 2009 and the purchase of the qualifying Canon EOS DSLR must occur between July 20, 2009 and October 5, 2009. The rebate form must be postmarked no later than November 2, 2009 and received no later than November 16, 2009.
You can see all of the details and get the rebate form from Canon here.