Using the latest version of the Uber app for iPhone you can estimate your fare before requesting a pickup. Just tap the "Fare Estimate" option after you've set your pickup location and enter your destination address.
Tap the "Fare Estimate" option on this screen:
After entering your destination address, you will be given a fare estimate. Note that fares may vary due to traffic, weather, and other factors. Estimate does not include discounts or promotions.