The reason that the STIF has declared InterRail invalid, may very well be a very practical one (this is my guess, I have no official information on the topic): many (but not all) stations on the Transilien network have restricted access, using entrance gates - similar to many metro systems in the world. That means you can't get on the platform if you can't open the gates. And trying to stick an InterRail pass in the 3cm slot of the machine is not really going to work...
But as Nico noted, Mobilis is a relatively cheap option (which will fit into the 3cm slot
). The maximum price, for zones 1 to 6 - which you will only need if you're going to visit distant sights such as Fontainebleau or the Brie region - is € 17,30 in 2010. Disneyland is in zone 5 (€ 13,65), Versailles in zone 4 (€ 10,15), and if you're just visiting the centre of Paris (zones 1 and 2), it comes as cheap as € 6,10. Note, however, that Mobilis is not
valid to and from the airports... a clever way of making sure that the American and Japanese tourists buy Paris Visite instead
. So if you are travelling to and from CDG or Orly, Mobilis is not the ticket for you.