I did a quick writeup on the Servocity channel slider. This is an excellent starting point for DIY slider projects. The one on the site is too small for larger cameras but can be easily expanded with additional channels to hold more weight:
More info here:
http://kokomonstrum.com/pages/servocity_channel_slider_motorized_camera_slider_kit_18.asp
More info here:
http://kokomonstrum.com/pages/servocity_channel_slider_motorized_camera_slider_kit_18.asp