you can chain regulators like that, as long as everything is within spec it will be fine (though I bet somewhere in there is mains current that you can use screw terminals to wire in a wall wart, that is what I do)
for 24 volt signals coming in off of a sensor or whatever in a machine of any sort the best way I have found is to use a Opto-isolator and a 317
an Opto-isolator is typically a 6 pin (or multiples of) IC, inside it has an LED, the other side it has a photo transistor, I hook the higher voltage to the LED and use the 317 in series to ground set up as a current regulator to not blow the led. The transistor side is hooked between 5volts and the AVR's input.
you could use a resistor to limit the LED current, but dumping 24 volts into a single LED and dissipating that power is going to require a decent size package, and what happens if the voltage changes (I have seen these things float all over the place depending on whats happening)
Regulators are a bad idea cause you need a fairly signifigant amount of capacitance to keep them steady, which might eat your signals and be too slow to react.
Resistor dividers are a bad idea in this environment cause again the voltages float everywhere so its not going to be enough or its going to be too much on any given day
Another option is to use a Zener diode with a resistance in front of it, but when I tried that in an industrial machine they were touchy about noise
Last option is what someone might bring up, the AVR in the arduino has internal protection diodes, they are AWESOME for supressing a hit or two, but ... LOL... I once misread some Zener's to be 4.7V and they were 24V, so I was dropping 24 volts + all the noise and spikes from switches, transistors, motors and relays into the chip
it lasted about 2 weeks then went ape shit, the big ole Opto-isolator + 317 setup has worked flawlessly for almost a year now connected directly to mechanical relays switching thousands of times a day (with some back EMF protection added)
to be honest its not all that big since I sent off for some 10$ PCB's at itead studio and use SMT parts
so theres my 2 cents spread out to 3 dollars worth of words, but one of my main functions is to design and make automated equipment in a small electronics plant.