I wonder if I should sell my Canon Pixma MX860 all-in-one while it's still in the box, and get a new AirPrint-enabled printer when they come out...
I would return the MX860 BUT NOT for the reason you stated. If you dig into it online you'll find that the 860 had a slew of problems, especially with Macs. So, Canon quickly released the MX870 which fixed all of the bugs and issues. That's why you'll find the 860 so cheap in many outlets. The 870 is a great printer. Wi-Fi function works flawlessly.
As to your question... I don't think you'll have any problems using it with Air-Print. Air-Print will not need any additional drivers. etc. I imagine that just about any Wi-Fi/Bonjour enabled printer will work with it. "Air Print-Enabled" will just be a marketing and advertising term. No way Apple is not going to release something that doesn't work with the millions of current Wi-Fi printers.