Logan Bonner, one of the designers for Wizards of the Coast, has stated elsewhere on these boards (in a smug, "I-know-a-secret-that-you-don't" sort of way, as I recall) that there are rules for familiars, and that they are known to a select few.
I concur with Kordeth that they will probably become official with the publication of Arcane Power.
I would, further, speculate that the basic mechanics will be similar to what we've seen for the bag of tricks, gray from Adventurer's Vault, in that there will be an abstract "familiar", with specific characteristics applied to it by type, e.g. snakes having a poison bite attack, et c.
Continuing to speculate, I believe that there will be a class of spells bearing the "familiar" keyword, that affect one's familiar in various ways. For wizards, this will probably include the ability to perceive through the familiar's senses for a short period of time, and the ability to use the position of the familiar, rather than that of the wizard, as the origin point of one's spells.
It is also quite possible that there will be familiar-focused paragon paths for both the Wizard and Warlock classes.
—Siran Dunmorgan