Sometimes we forget or forget to look after basic SEO topics. The other day, I caught one of my programmers allowing both capitalized URLs and lowercase URLs result in having the same destination page. Let's take Twitter as an example, since it was brought up in a Google Webmaster Help thread.
Twitter allows both capitalized and lowercase URLs return the same page. For example, both http://twitter.com/google and http://twitter.com/Google return the same exact page, content and information. But Google considers http://twitter.com/google and http://twitter.com/Google to be different pages, in many cases.
Now, Google isn't that dumb, just do a search for http://www.google.com/search?q=http://twitter.com/Google (the capitalized version) and Google will know you really want the lowercase version:
But you don't want to make Google figure this stuff out. What you should do, is make sure there is only one version, preferably the lowercase version in my opinion. If someone does go to a capitalized version, 301 that page to the lower case URL.
Forum discussion at Google Webmaster Help.