Note additionally that 8051's look at the interrupt inputs once each machine cycle. So an input that works in transition mode must be high or low for at least 1 machine cycle to be recognized in that particular level.

You should download the '51bible document from the LINKS page at the left. There is a goodly amount of information about this subject starting on page 3-23.
Here is the link to the document itself....

http://www.intel.com/design/mc...238302.pdf

Michael Karas