2008 worldcon site selection|
Nope, not talking you out of it! I think Denver would be fine; it's more central for the US as a whole and would give the out-of-the-country people a chance to see something other than a coast. August in Denver is likely to be more pleasant weather-wise than August in Chicago, not to mention the fact that it's beautiful out there.
Except Chicago is indoors and you do not have to emerge to go to restaurants until later in the day when it is cooler.