1) Acting upon release instead of press will drive the users mad, I know of sales lost for that reason.
2) The LPC932 has a keyboard interrupt feature, forget about nand gates, schmitts etc.
3) Do not use the keyboard interrupt or (if not a 932, the nand interrupt) as anything but a wakeup interrupt, if you do not put your processor to sleep, there is no reason to use a keyboard interrupt (see 4))
4) set a timer to set a flag on interrupt (I set the timer for 10ms, works with all kbds I have used). In the main, when the flag is set, scan the keyboard and if the result is identical to previous scan, the result is valid.

Erik