According to Eurogamer, the next Tomb Raider title will come out in Holiday of 2011.
Although Lara Croft and the Guaridan of Light is still in development, the new title is being made by a US developer Crystal Dynamics who are currently exclusively working on the title.
Crystal Dynamics has two teams, one of which is a small team who are working on the downloadable Lara Croft and the Guardian of Light, which is expected to be released later this year, while the other and bigger team is working on this unannounced Tomb Raider title.