Although the octopus is killed instantly, the nerve endings remain very active and cause quite alot of residual movement and suction in the tentacles. Some restaurants post warnings to chew well as this phenomenon can pose a choking hazard particularly for those who may have had one too many cocktails! Other more experienced diners purposely don't chew as much since they enjoy the squirmy sensation.
I'm a pretty adventurous eater and I really enjoy a quality sushi/sashimi meal, but I think I might draw the line here... but you never know after a few drinks....
Check out this video if you're not too squeamish!
http://www.flickr.com/photos/yusheng/2400250326/
Now on the other hand, some quickly grilled octopus marinated in olive oil with citrus and spices, and a nice frosty malt beverage....awesome!

