from posts like this http://www.8052.com/forum/read/174600 the implication is that a high priority cannot interrupt a low priority interrupt?
a) an interrupt set to high priority by the IP SFR can interrupt a low priority interrupt.
b) an interrupt of equal priority by the IP SFR can not interrupt an interrupt of equal priority by the IP SFR
c) the 'priority list' (polling sequence) in the datasheets/bible only applies if the interrupts occur at the same time and will NOT result in an interrupt interrupting another

8052.com said:For example, you may have enabled Timer 1 Interrupt which is automatically called every time Timer 1 overflows. Additionally, you may have enabled the Serial Interrupt which is called every time a character is received via the serial port. However, you may consider that receiving a character is much more important than the timer interrupt. In this case, if Timer 1 Interrupt is already executing you may wish that the serial interrupt itself interrupts the Timer 1 Interrupt. When the serial interrupt is complete, control passes back to Timer 1 Interrupt and finally back to the main program. You may accomplish this by assigning a high priority to the Serial Interrupt and a low priority to the Timer 1 Interrupt.

Erik