rebates are applicable above and beyond any other money you negotiate from sticker - they should not be included as part of the dealer's discount. they're separate discounts - rebates come from the manufacturer to the dealer, so it's not like they're making less money on an MSRP deal vs. an MSRP-minus-rebate deal.
as to rebates - they vary month-to-month so there's no telling whether the current $3k will be renewed or not. given the current economy, i'd be surprised if overall rebate packages decrease much, but if they do, give 'em time - they'll have another sales push and bump 'em up again at some point sooner rather than later. if nothing else, late summer usually brings end-of-model-year closeout pricing that you could take advantage of if you're willing to wait.
also, there's more mark-up in a loaded model than one with very few options, so you should have more negotiating room accordingly. my suburban is a loaded LT3 Z71 and we got it for true invoice less all rebates ($2250 at the time - early november '07). in looking at invoices for different models, the difference between MSRP and invoice is much larger on, say, a loaded LTZ than on a base LT1. check edmunds.com for ballpark invoice pricing - in my case they were about $700 low, but at least it gives you a starting point.
if you get on chevy's website you should be able to check inventory on multiple area dealers fairly easily - that's what i did to find mine. had to dealer-transfer it from georgia because it was the only one in this color in the southeast with all the options i wanted at that time, but custom-ordering is certainly another viable option. theoretically you should be able to get just as good a deal on a custom order as an in-stock unit, if not better, since the dealer knows the truck won't sit on their lot eating up floorplan money or inventory space - as soon as it comes in you buy it, thus maximizing their profit buy not having it sit around waiting on a walk-up buyer. on the other hand, the dealer would certainly rather unload one of their current units than sell you one that won't come in for a few weeks - so that may create more incentive to give a better deal on an in-stock unit if you can find what you're looking for.
at the end of the day, though, if you're not finding a dealer willing to come close to invoice then try another one. gas prices are only going up, so the full-size SUV market appears to be a buyer's market - i believe there are a lot more units on the ground than there are buyers right now, so good deals should be out there in abundance.
