Take these with a grain of salt, but these are some rumored rules I've heard about a 2012 upcoming Eldar book!
Wisdom of the Ages - Eldar Farseers are masters of controlling psychic powers. Any psychic test roll of 10 or less made by Farseers automatically passes unless other modifiers negate this, especially anything by forces of the Imperium.
Made Of Freaking Glass Or Something - Eldar are frail and break if glared at harshly. All enemies re-roll rolls to wound against Eldar models. Enemies that would normally get a re-roll to wound get a second re-roll, overriding the "one re-roll" rule completely.
Ninja Throwing Star Gun Pew Pew - Eldar shuriken weapons are especially deadly against unarmored foes. Any enemy model with no armor save or invulnerable save wounded by a shuriken weapon automatically takes one wound.
Flying Tanks! - All Eldar vehicles use a "bottom" armor characteristic of 6 instead of front, side or rear when targeted by anything on the ground. Grav tanks that have disembarked troops that turn and have not moved since are exempt from this rule until they move.
I'm Pretty Sure I Have Some Art To Do - Eldar guardians can always choose to be wiped out in assault, since they'd really rather be pursuing other interests.
Ancient Enemy - An opponent fielding any unit aligned with the Chaos god Slaanesh may touch the Eldar player in a sensual and potentially confusing way immediately before each Eldar shooting phase.
Alright! Time to start saving my pennies for a new army!