Many of the newer N-Series mobiles of the last year have this feaure. There's also some gesture-based software from a 3rd party company I forget the name of that enables this with other Symbian devices which have an accelerometer.

When its in your pocket, to ignore, do a hard double tap on the back of the case. This will simulate the idea of fliping the device and ignore the call.

The N97 does more of a silence the call vs an ignore though. Usable, but not as fun IMO.