It is on page 62 under Operation environment:
"Make sure that the device.....
...
. is kept at least 30 centimeters (one foot) away from a mobile phone."
This sounds really hard to believe, that having a mobile phone within a foot of the tablet would be harmful. Heck, they make a version of the A500 that has 3G! I can't figure what they must think could happen, and I can only find ONE similar mentioning of this on an old Apple forum post from 2008, where the guy said a store rep told him to keep his cell phone away from his Macbook. Other than that, I went 6 pages into a Google search and found no explanation at all.
Before reading that today, I had been placing them beside each other to charge on my desk, and even sat my phone on top of the tablet's closed case to carry them once. I haven't seen any issues so far.