I saw it mentioned in this forum how someone lost rank when they rolled out a rebuilt site that had the file names but the case was different like /about-us.asp and About-Us.asp. Apparently Google viewed them as different pages so would it be advantageous to publish with both versions? I am guessing that it would hurt eventually and may not have much value.
Always use lowercase. Better for type ins. If you start making multiple pages with the same content, you'll also run into duplicate content issues.
I don't think they lost PR due to the case of the folder/filename, but more that the PR established was tied to the old path/filename. Now the new files have to start over. Google is case-insensitive for searching, but path information is not. It can't be due to uri specifications and file systems.
I agree. Go with lower-case file names and use dashes between your words. This is more of a usability benefit but the search engines will appreciate it as well. If you switch the case from upper to lower, you may want to consider doing a 301 redirect just to be on the safe side.
The keywords are case insensitive. The file names and paths however, are. If the keywords are in your page's content, then I wouldn't worry about it too much.
I rarely see sites with upper case urls and even when I do it is an amatuer site...I can't see people at their keyboards capitalizing letters when they are searching so I would say case doesn't matter I just did some searches are the same results are returned for lower and uppercase