Really cool video ....some amazing precision flying !   

http://www.snotr.com/video/4391

Awesome    

Just found out....the Pilot is Bob Hoover......he is often known as the Pilot''s pilot in aviation circles !

http://en.wikipedia.org/wiki/Bob_Hoover

why water from the glass didn't spilled out? ........any explanations?

The centrefugal forces didn't let it !
See this - http://en.wikipedia.org/wiki/Centrifugal_force

 ..How can I forgot such a basic concept  ......., btw Thanks a lot

I guess just the way the pilot doesn't fall off his seat when upside down....the same physics applies to everything else in the plane...isn't it?

Nope, Pilot didnt fall because he is strapped tight on his seat.... ....BTW does the centrafugual force in a barral role is strong enough to hold the water from spilling  ......in a barallel role If I am not wrong horizontal speed is much more as compare to vertical speed and in this case its quite evident that the role was not so quick .......so still Centrifugual force ??