Carl, I think I worded my last post wrong, what I meant was
The timer overflow interrupts used by the Arduino core to keep track of millis may cause higher interrupt latency for the INT0 external interrupt. Also there are Arduino core functions which disable interrupts globally such as the microsecond delay functions.
V-USB requires low interrupt latency and also for the interrupts to be enabled all the time. Hence I think I need to modify the Arduino core so that it is friendly with V-USB
trialex, you can't use the serial monitor without connecting a serial port or a serial port to USB converter.
I hope all Arduino users understand that the serial port functionality usually comes from a built-in FT232 chip, a FT232 cable or similar, or the ATmega8U2 (on the Uno board) which does the exact same thing as a FT232.
I did not include this FT232 because it would add to the cost and size significantly, and many Arduino derivatives do not include this chip or cable.
You are free to purchase an FT232 cable separately in order to use the serial port monitor, but unlike Arduino, it is not a requirement for USnooBie to be programmed.