@imike I'd guess most people
perceive they sweat more when their clothing is wet as opposed to to wet bare skin.
@Dave1 The Under Armour you speak of does work!
I've been able to hike
much more comfortably in triple-digit heat wearing a white Under Armour long sleeve heat-gear shirt than bare skinned. Reflecting the sunlight and cutting UV rays can do quite a bit to allow our bodies to breathe. I found the sweat was drawn off my arms into the sleeves and particularly with a breeze it evaporated fast enough to feel cool.
When I first moved to AZ in '02, I either hiked/biked with a tank top or no shirt until I got enough experience to have built up a database of sorts as to what works best for what conditions. As much as I sweat here, it wasn't any more, and in many cases, less than back in MN. High humidity in warm/hot weather was the killer. In the winter it was a matter of taking layers off fast enough to prevent excess sweating, and of course the re-layering quickly as you wind down the exercise. When mountain biking in the winter months (yes, I biked in the snowy months) I would dress very lightly, knowing that within 5 minutes I'd be plenty warm. As long as I didn't stop for break more than a minute or so, I wouldn't get chilled.
Now after living here long enough, for mountain biking in the summer I'm more likely to wear a very thin biking shirt because most of the time I'm moving fast enough for the moisture wicked into the shirt to be evaporated quickly enough I actually feel quite comfortable. For me I find it a bit weird my body seems to be able to regulate its temperature more efficiently in the higher temps than in cooler weather.
I will still go without a shirt from time to time (to keep that nice bronze AZ tan and absorb all that Vitamin D) while mountain biking. But for hiking whether I'm actually perspiring more or not, in my experience I
feel better by using the Under Armour heat gear long-sleeve in the highest temps (105-110) and very thin convertible pants, the best being the ones that have a side panel below the knee that can be unzipped to promote more air flow. For 95-105 I'll usually hike either bare armed or a light colored long-sleeve when going through brushy areas.
As far as experimenting... to come up with anything concrete, it would need to done under at least semi-controlled conditions. Without some scientific means to quantify the tests what may seem valid to one person may not be to another. Again,
perception is the key word here... I don't doubt that even our state of mind from one day to the next will make a difference of our perception of how much more/less sweating we did based on what we wore.
@teva joe Back to your 20 mile no-pack scenario... there's quite a number of variables that could affect the results that need to be the same:
1. Need to hike the exact same path
2. Temperature
3. Humidity
4. Amount of sun (preferably clear)
5. Particulates in the air, pollen, dust, smog, etc. (that's why the more closed and controlled a test environment can be made is what determines the accuracy of the result)
6. What you ate and how much you exercised the day before, the night before, the breakfast you had...
7. Your mental and emotional state (... maybe you had a very stressful day before one hike but not the next)
8. Add your own thoughts here...
If the testing were completely scientific, tracking how much air intake, etc. #7 may not show up quite as much as if it were just a perception test. Based on perception alone, I'd hazard a guess #7 may play a pretty significant part.
Like foot gear... it will come down to what works for us based on our own experience. But, Joe, if you do your 20 miles with no water I'd love to hear what you find out...
What was that?!!.. no water! 
Well, wouldn't that be the most accurate test? Otherwise you'll have to make sure you sipped the same amount of water at the same points on both hikes, wouldn't you?
) I suppose it may be evaporating too but my shorts aren't getting soaked