Basic lathe threading support (spindle synchronized motion) is now in EMC2. The image shows a 3/8-16 thread that was cut with EMC2.
The G33 syntax is subject to change. Right now, the usage is:
G33 X- Y- Z- K-
where K- gives the distance moved in XYZ for each revolution of the spindle. This syntax is subject to change (in particular, to use F- instead of K-). Support for canned cycles may be in store--more about this is at Lathe Code
The [sim/lathe] sample configuration has an emulated spindle encoder, and the example program [threading.ngc] shows G33 usage.
Online documentation for G76 (canned threading cycle): http://www.linuxcnc.org/docs/html/gcode/gcode.html#sec:G76-Threading-Canned
Online documentation for G33 (spindle synchronized motion): http://www.linuxcnc.org/docs/html/gcode/gcode.html#sec:G33-Spindle-Sync