You can buy from Amazon a day or two before the rebates expire and select the free super saver shipping which takes a few days to process and if the rebates are extended, cancel your order and then reconsider which route you want to go. If they aren't extended, then at least you were able to jump in on the deal.
Thanks again guys, and this is great advice on the rebates. After spending far too long reading the endless debates over the 70-200 f/4 IS vs the 70-300 f/4-5.6 (and determining that both are really excellent) I'm going to follow the above advice and order the 70-300L via amazon while the rebates are intact. As a hiker I'll be outside a lot and think I'd appreciate the compact size and newer IS more than the downside of the variable aperture. Sounds like you can't go wrong either way.
I'll then start hunting around on ebay and craigslist for a good deal on a whitebox 24-105. With the current kit pricing on the 6d I'm hoping I can get one in the $700's. We'll see. Maybe that way I can pretend I didn't make a mistake not getting it at first :-)
And Niterider - I would love to see some of your photos from the 14mm samyang. Not going to get it now but would be interested down the road for sure.