Amazon has been matching price drops from WM and Best Buy pretty often with various BR titles. We've seen Amazon drop prices on against both WM and BB price drops. Currently, BB has Up for $22.99, so Amazon is matching that, and when it had dropped previously to $24.49, Amazon matched that as well. Once they see that WM has it at $19.96, I'm sure they'll drop it to match WM, or come close at $19.99.
Monsters Inc is $34.99 at BB, though I'm sure that's advance of the sale starting next week when it comes out. WM has it cheaper than Amazon to get more pre-orders and Amazon will respond in kind. I'd be surprised if they didn't lower it to $22.99 to come close to WM's price.
Either way, as long as we stick with the pre-orders at Amazon, we should end up with as cheap as we can get. The BB sale where you'd get the $4.99 stuffed toy dog w/Up and save $10 might be as cheap as you can get for Up by itself. Assuming it sells in-store at $23.99 per the ad, you'd get Up for $18.99 + tax. Right now, I'm getting Up for $17.99 w/o tax and shipped to my door, though I'm also buying Monsters Inc for $19.49 w/o tax.
I think if you were just going to get Up, then the bundle purchase with the stuffed dog works out best. If you were going to get Monsters Inc as well, then the Amazon deal w/$10 bonus savings is the better deal.