1. The tablet should be already upgraded to ICS 4.0.3[LEAK] for A500.
2. Download this archive. Unpack it to any place.
3, On a tablet: "Settings" -> "Applications" -> "Development" -> "USB debugging" switch on.
4. Сonnect the tablet to the computer.
5. Open the directory with the unpacked archive. Execute file: for windows - runit-win.bat; for Linux - runit-linux.sh.
The root is received!
6. Now you will have to install the following programms:
Don't install Busybox! It is already installed and established! If you reinstall it, some programs will become unable to access root permissions!
This method uses the 'mempodroid' exploit and some workings out by ZeroNull and vdsirotkin (4pda.ru).
PS: Command "mount -o remount,rw /system" for directory /system (partition) - works perfectly!