iMac 10.6.8 pointing fail with Bluetooth enabled
Problem solved!

First, the underlying cause MAY have been an unusual circumstance: a smartphone (Galaxy Android) was placed next to the iMac, turned on and with Bluetooth enabled, and left for hours to charge.  Could this proximity have resulted in the Mac getting some strange Bluetooth input?

Fix:  Using the temporarily-installed mouse and keyboard, I removed the existing Bluetooth keyboard and trackpad entries. (I just removed every Bluetooth device.)  Then I used the standard procedure to re-pair the wireless keyboard and trackpad.

After removing all Bluetooth devices, I could not get the Mac to detect any Bluetooth devices until I restarted the Mac, at which point device detection and pairing worked flawlessly.  

My best --and admittedly ignorant, sketchy-- theory:   Some mal-formed event got stuck in the Bluetooth input stream, causing the trackpad up and/or down states to be hyper-sticky.  That event couldn't be cleared without removing the originating device and re-pairing.

HTH