I'm trying to send 24V AC to an Arduino digital in. I have a separate power source, sorry for the confusion.
As stated before, you can't do this directly. You know this.
My question about the wall warts was whether the output was just a function of the input or if it was only a specific voltage. If input is 120V AC and output is 6V DC, would 60V AC yield 3V DC? if that's the case, 24V AC would yield a little over 1V DC.
It really depends on how the wall-wart is designed; if its one of the newer switched-mode supply wall-warts, then no, it won't work. If it is a linear-regulated wall-wart, then it probably wouldn't work, either. If it is a linear un-regulated wall-wart, it may work, depending on its design.
What would be better would be to remove the transformer from such an unregulated wall-wart, then build your own step-down rectification system, then place a small regulator after that (with appropriate capacitors and such - they -are- needed). The regulator would need to be matched to the output of the transformer, so that it will work (for instance, if you were using a 7805 for the regulator, then your output of the rectification would have to be 7 volts or greater, because the 7805 has something like a 2 volt dropout, IIRC).
Something you could try, though, would be to detect voltage indirectly by a coil around one of the 24 VAC wires. Make a small coil on a form (like a soda straw or a piece of a plastic ball-point pen body), wrapping say 100-200 turns of 20-24 gauge wire around it. Laquer it up or otherwise seal it. Pass one leg of the 24 VAC line through the coil. When it is "on", the coil should generate a low voltage that you can measure. This voltage will be AC, too, so you'll want to rectify and filter it with caps, depending on its amount, you might need to rectify it, but it should be detectable - you would have to experiment with it.
I think I'm actually going to go with the rectifier + capacitors. Do I need the voltage regulator if I use a voltage divider?
If you are going to use a rectifier, try to find a 2:1 step-down transformer to place before the rectifier, so that you turn the 24 VAC into approximately 12 VAC, then into 12 VDC - then use a 7809 regulator (with appropriate caps) to regulate that down to 9 VDC, then a 7805 regulator (with caps) to take it down to 5 VDC; I would then use that to drive a small 4N26 optocoupler (put a resistor in place to limit current to the LED), and the output of the optocoupler to signal the Arduino (don't couple the grounds - this will isolate the Arduino from the AC side completely).
You want to take these steps (12 VDC->7809->7805) because if you just went with 12 VDC to a 7805, the excess voltage (7 volts) is dumped as heat from the regulator, and you would need a large heatsink to keep from frying the regulator. By doing it in stages, you don't have this heat issue.
If you wanted to spend the money, there also exist miniature switched-mode regulators to replace a 7805, that can easily take high voltages and convert them down to 5VDC (they also come in other values) - they aren't cheap, though; a 7805 replacement device costs around $15.00.
Another possibility would be to hook up a 24 VAC SPST relay in parallel with the output you want to monitor, then use the contacts of that relay to switch an input on the Arduino to ground (activate the internal pull-up first). That would be simple and fairly cheap (if you can get the relay).