Using CSR8510 A10 based Bluetooth 4.0 (Low Energy) dongle with mid-2011 iMac
Oh snaps, I'm going to make you happy @ThomasW.

Okay so after you do the HCI bootmode thing? You also need to whack /Library/Preferences/com.apple.Bluetooth.plist and bounce your system.

If you have both internal Bluetooth and this adapter you may also need to use Apple's "Bluetooth Explorer" (via developer.apple.com no ADC subscription required, get the "Hardware Tools" bundle) and pick a different device.

Just got this working tonight via the change to bootmode and subsequent removal of the existing plist and my Magic Trackpad is totally happy, will test other devices later, it's been a long evening.