For over a decade, infantrymen and commandoes have been buying expensive, high tech, thermal underwear to keep them warmer, or cooler. This stuff has been around, in various forms, since the 1970s, but has been getting more capable in the last decade. In Iraq, Many soldiers and marines currently buy "hydrophobic" t-shirts (like Coolmax and Polartec), which are able to wick the sweat off the skin, while also keeping external moisture out and keeping the troops cooler and less liable to heat stroke. 

 The U.S. Navy is now buying this stuff for their SEAL commandoes. They are getting the InSport Polartec, which has an additional feature, silver microfibers that have an anti-bacterial effect. Among other things, this reduces body odor a lot if you have to wear the t-shirt for a week or more straight. This results in healthier, less miserable, troops. Normally, the government doesn't like to buy $50 t-shirts. But the troops will, and eventually the SEALs won't be the only ones that have their combat quality underwear paid for by the government. It may seem like a minor thing, unless you are stuck in a combat zone for several weeks without access to a change of underwear. 

The Navy won't be buying this sort of clothing for it's sailors. That's because the high tech fabric melts when exposed to the kind of heat encountered when a ship suffers battle damage. In general, crews of ships, armored vehicles and aircraft avoid these fabrics for that reason.