B&H Photo Video now has the Canon EOS 5Ds in-stock at $3699.00 — their site lists it as available to ship tomorrow (Monday). They’re also offering a 2% reward. Amazon is also showing it in-stock.
There’s also a new stackable mail-in rebate: when you purchase lenses at the same time as the EOS 5Ds, you can effectively double the existing rebate. You can find the rebate form here. It clearly says that this new offer “may be combined with other Canon promotional offers running concurrently”.
You are eligible for the double mail-in rebate with purchases at street price, please inquire for details.