hi Radu;
Printf shall basically output the charecter on serial port.you have set t0 priority to be 0.Therefore it is at the same or lower prio than ext0.In either case it is being initiated from EXT1 isr.it means untill you finish your EXT isr no other isr of same or lower prio shall be executed.It seems your printf is putting more than 10 charecters.By 9600 baud(just an example..) it shall take more than 10 miliseconds to execute and hence it is unlikely that you can get 1 milisec isr to work.may be that is the problem.
Also are your counts for TH0 and TL0 intercahnged???

Pranav