Hot weather(70-100F) shirt

I would agree on the fly-fishing shirts, especially in a light color, but the Rail Rider Eco Mesh long sleeves are lighter, no buttons or pockets. I wore one everyday along the JMT in August. Worked great. Dries fast.

Check the photos at Rail Riders:

http://www.railriders.com/photo-gallery-running-adventures-g-4.html

do you guys think a tech t lite merino (the lightest icebreaker) would be too warm for the 90's?

Yes, without a doubt. Then again, almost anything is too hot in the 90's. I like the proposals for light cotton in the GC summer situation, even though I initially said capilene 1.

Thanks for all the ideas. I currently jump back and forth between liking a cotton tee and liking a wicking type shirt when I am hiking in hot weather. I'm also in AZ and like both. The cotton keeps cool and holds water longer. The wicking type shirts keep me feeling less sweaty, which I like. But are not quite as cool. I wanted to see if there was something better out there. I picked up a capilene 1 short sleeve and I'm going to try that out for comparison.