It appears that whenever I reboot my Mac Pro, the Lightroom 2 "64-bit flag" gets reset back to 32 bits.
So, I deselect the Get Info checkbox "Open in 32 Bit Mode". As soon as I reboot, the checkbox is selected again.
This is a 2-month old 8-core Mac Pro, 10GB RAM, OS X 10.5.4.
Any suggestions? Thank you all.