Can you tell us why you CAN NOT use a isolation transformer ( a cheap take-apart AC Wall adapter ) And why you want a DIRECT way ? So what is the "issue" here ? My opinion is : Risk to damage the Arduino <--- BIG TIME !!!
Why is a PC transformer an option but a wall wart isn't?
Are you trying to sense supply anomalies (interruptions, brownouts, spikes, RFI, etc.)?
Your main problem of using a simple resistor divider network is that your circuit will be either at near ground or near mains voltage, depending upon the "polarity" of your input wiring. Hook it up wrong and your whole arduino circuit is potentially lethal.
How about this, it is the same regardless of input.
n.I am looking to monitor a generators output 120/208v & 120/240v