These are the more reasonable rumors related to Tau that have been floating around the internet.
A new codex is in planning, likely slated for 2012, possibly 2011.
Crisis Suits and Broadsides are getting new models. The crisis suit update has apparently been completed for quite some time because GW was unhappy with the fragility of the model (particularily at the ankles). It's possible that the Crisis Suit model will be released before the next codex update, but GW will have to work it into their busy release schedule.
Several Tau models are no longer able to be ordered.
A higher-up at GW stated long ago that, "with every Tau release they will get new allies." There was a sighting of a centaur-like model in progress at GW in 2009, which led to rumors that a new Tau codex might come with Tallerians as allies. The Lexicanum on Tallerians: "Tarellians are a minor reptilian species of alien found throughout much of the galaxy. They are commonly known as Tarellian Dog-soldiers, because of their snouted faces and their habit of working as mercenaries. During the Great Crusade the Imperium virus-bomed most of their home worlds, almost wiping the species out. For this reason they have a great grudge against the Imperium. The Tarellians have not been conquered by the Tau but are often found fighting as mercenaries in their armies, particularly when the war is against humans."
Commander Shadowsun is being either removed or completely reworked, as she was intended as a special release for Cities of Death
The new codex will include a large number of new named characters
(btw, apologies for posting in the wrong forum. The forum bug wouldn't let me post in the News&Rumors forum)