Quoted from the 1.8h release notes:

bug: fixed 'focus-follows-mouse' weirdness when click-drag on 2nd screen window (keeping you out of a window)
Sadly, this still doesn't work for me. Only one display, 2560x1600, four windows (1280x800 each), Vista x64 Business, Geforce 8800GTX with 174.74 drivers. When click-dragging, I still "lose" the window. This sure is a frustrating bug, I hope you have any ideas left what might cause it.