I've now experience twice on my MacBook Pro 6,2 with 12.04 and Unity, where some combination of two or three fingers on the track-pad causes almost all keyboard input to be blocked, and the the mouse-cursor is locked in the state with the closed hand as if I were dragging on the move window icon after having done a three-finger tap.
The mouse still moves, and I can do alt-tab to switch apps, but no other keyboard input is accepted, i.e. esc doesn't work, and click the mouse doesn't work except to activate other windows (you can't select menu options etc). So even when the terminal window is activated, I can't type in it. The only way out was to reboot by holding down the power key.
Has anybody else experienced this? What's a work around?