Based on the Intel
roadmap Socket 775 has about 1-2 years to go before it will be considered obsolete.
Currently Socket 775 supports the Core family of processors (65 nm) and the recently introduced Penryn family (45 nm). Penryn will be mainstream going forward in 2008.
By early 2009 the Nahalem family of processors will replace the Penryn. The Nahalem will feature an integrated memory controller, thus will require a new socket type.
This
Anandtech article says that Nahelem ramp production in late 2008 and will reach full production in 2009.