How soon until you "really have to have it"? Can you wait 5 weeks?
I realize that this is speculative, "but"...
My reason for the question is because the 70-200 f/2.8 IS has been included in all of the recent rebates that Canon has run, so if you can wait, I'd say that the odds are probably very good that another rebate's going to start pretty soon...if past history holds, on October 15th (5 weeks from now).
In doing some digging around, the last several advertised rebates from Canon have run:
10/15/2004 - 01/31/2005 (triple dipping)
04/01/2005 - 07/15/2005
10/15/2005 - 01/31/2006 (triple dipping)
04/01/2006 - 07/15/2006
This pattern suggests:
10/15/2006 - 01/31/2007 ... maybe with triple dipping.
What I mean by "triple dipping" is that Canon allows the rebates to increase (up to triple) if you buy multiple eligible products.
For example,
here is this past spring's "normal" form, and
here is last Fall's "triple dip" form.
For example, I got some stuff last fall when Canon was running a "triple dip" promotion: the 20D had a $100 rebate and the 70-200 f/2.8 IS had a $50 rebate, but by buying them at the same time, I was looking at getting a 2*($100) + 2*($50) = $300 rebate.
But since maximum leveraging is with three items, I ran the math and decided to add a Canon ip660D printer,. This was a $150 printer with a $25 rebate, so what had beena $300 refund became a 3($100)+3($50)+3($25) = $525 refund. More importantly, it was a $375 refund after the cost of the printer was subtracted, which effectively meant they gave me a free printer and an extra $75 rebate,
because I was willing to loan them a little more of my money ($150 for the printer) for 2 months.
As such, my suggestion is to start to looking to see what else you might want to pick up (and can afford within your budget), as well as starting to ask friends/family if there's any Canon goodies (likely to be on the rebate list) that they might want to go in together on an order so that you both save some extra bucks.
-hh