The solution, at least for me on my system, was to go to MIDI Sync (in Preferences) and set all my Control Surfaces that use "USB Audio Device," and their input/output to 'none." On my system, I have several devices that show up as "USB Audio Device," including an EWI USB, Beri BCR-2000 and Korg nanoKontrol (if I recall correctly).
After setting all Control Surfaces and their input and output drop-down menus to "none," I selected APC40 as my controller, and USB Audio Device as input and output. Then, set the APC40 "track" and "remote" entries to "on." That fixed it, and now Live 8 and the APC40 seem very happy together. I'm pretty happy too!