After taking my father-in-law to the Columbia Employee Store, he picked up several lightweight LS shirts. Columbia doesn't list weights on their website, so I took the chance to weigh three shirts (all in size medium, with tags removed) and the lightest was the Tamiami II, from their PFG fishing line.
Weight is 6.8 oz, which is the same as Rail Riders Eco-Mesh shirt, but with slightly more conventional styling.
UPF 40, too.
Those looking for a LS sun shirt might want to check this one out.
http://www.columbia.com/mens-long-sleeve-shirts/men-shirts-longsleeve,default,sc.html?sz=1&start=14#
