That's a good question and sadly the "summer swelter" is most definitely underway, with two record-breaking days heat-wise this week already. The good news is that it's about to break and should be back in the 80s by the weekend, so the 100-degree+ temperatures we've been experiencing of late are a prelude to what's to come, but we're not quite locked in to the summer misery yet.
The reason why meteorologists record the temperature as taken in the shade is because what they need to know is the temperature of the air itself, not of objects that have been warmed directly by the sun, nor of the heat that they're radiating back into the surrounding air. The temperature of the air affects its stability and the chance of precipitation, for example, so it's crucial for many calculations. As an example of how disparate the actual air temperature and a thermometer's recording can be, while researching this answer we came across a photo of a USA Today weather editor standing by a thermometer at the National Science Foundation's Summit research camp on Greenland's Ice Sheet, about 10,000 feet above sea level. The temperature on the thermometer reads 80 degrees, because it's being hit directly by the sun; the actual air temperature was in the 20s, but the weatherman was wearing no hat or gloves because the air was still and the strong sun was warming him, too.
In addition to being situated in the shade, official thermometers are located on "native" soil (as opposed to asphalt, concrete, grass etc.) and contain an aspiration fan that keeps things moving so that hot air can't accumulate unnaturally and skew the true temperature. However, as you point out, the official air temperature in Las Vegas right now is less than 100 degrees, but that bears little relation to what you'd be experiencing walking down the Strip in the afternoon sun today, where you're surrounded by asphalt, steel, brass, glass, and all kinds of other surfaces that are pumping the heat they've absorbed back into the air around you.
Hence, even if you see the temperature displayed outside a bank or some other building, it depends on where the thermometer is located -- at ground level, on the roof etc. -- as to what temperature it's actually telling you. The same goes for your car thermometer, although the latter should give you a pretty accurate guesstimate of the temperature outside.
In an attempt to answer your question, we called our local NOAA office, who we thought might have some information. However, although very amenable, the gentleman we spoke with was unable to help us much since, he explained, there are just too many variables involved (surfaces, wind speed etc.) to hazard a guess at actual temperatures or to come up with any kind of formula for calculating how hot it really is. The best he could tell us was that anything over 110 degrees is "stupid hot." That's official.
From some other related reading we did, we would suggest that if you're somewhere like the Strip on a still day in July, you can probably safely add another 10-15 degrees to whatever the official temperature is; so today, when it's currently officially 97 degrees Fahrenheit, it's probably 107 degrees or more outside Harrah's or the Riv.
Plus, another interesting factor is how hot it feels, which is also affected by the relative humidity. There's a Heat Index Calculator on the NOAA site, where you can put in the current temperature and humidity and it will tell you how hot it feels. We just tried it for today's stats -- 97 degrees with 9 percent humidity -- and it informed us that it would feel like it was a mere 91.9 degrees.
