Detecting voltage in analog pin above a certain level with interrupts

I need to detect when the voltage reaches 2.4V or more in an analog pin, and polling is not an option

I've read about the analog comparator black magic, which would make sense for an arbitrary threshold.

But in my case the threshold is around 2.5V, a little less, which is half 5V.

Would this line do the job?:

attachInterrupt(A0, someisr, RISING)

What voltage level or levels would trigger the ISR? Will it be running as long as V(A0) crosses that level, or will it stop after being run once?

Have a look at this excellent write-up on how and where to connect and use the comparator.

Would this line do the job?:
attachInterrupt(A0, someisr, RISING)

No, but you can use a pin change interrupt, which will signal a change that crosses the threshold for digital 1/0. http://playground.arduino.cc/Main/PinChangeInt

buffer_overfly:
What voltage level or levels would trigger the ISR? Will it be running as long as V(A0) crosses that level, or will it stop after being run once?

A pin change interrupt is triggered when the voltage changes between 0 and 1, in either direction. The voltage level that a 0->1 transition occurs isn't well defined. Atmel guarantees level 1 at a minimum of 0.6Vcc, or 3V for a processor running at 5V. But it will likely be something less than that. If you want a more well defined threshold you should use the comparator to generate an interrupt. That's what it's there for.