Permethrin -- http://en.wikipedia.org/wiki/Permethrin
As it states, it is a neurotoxin. Personally, I would rather be close to DEET instead (although DEET can melt your clothes). Pick your poison.
Actually, I just hike with a wind shirt (and bug net, bug gloves, etc.). I've worn a bug shirt without a T-Shirt underneath, and it is OK. I generally just wear my T-Shirt all of the time instead, so I really don't save any weight by leaving it at home (I don't feel like walking around half naked and I wouldn't want to wear a wind shirt all of the time).
So, I see what you are getting at with your original post. It would be nice if you had a base layer that was as comfortable as a T-Shirt, but bug proof. Maybe it could have removable sleeves (or sleeves that are easy to roll up). The Original Bug Shirt (http://www.bugshirt.com/) is made out of cotton, but it is not comfortable to wear and is fairly heavy. It is kind of baggy (for good reason). I only break it out when I know I'll see heavy bugs and I'm going on a day trip. In general, in order to make a shirt bug proof, it has to have a very tight weave. I could see a synthetic T-Shirt being made that way, I just don't think anyone has done that (and even if they did, it wouldn't be close to the weight of a wind shirt). Plus, if you had that, it wouldn't breathe that well (another trade-off).