I like to have my Computer at the top of the Sidebar. Here's how using Terminal and TextEdit.
Convert ~/Library/Preferences/com.apple.sidebarlists.plist to XML and open it:
plutil -convert xml1 ~/Library/Preferences/com.apple.sidebarlists.plist
open -a TextEdit ~/Library/Preferences/com.apple.sidebarlists.plist
Find Computer. It should look something like this:
<string>Computer</string>
<key>SpecialID</string>
<integer>8399481092</integer>
<key>Visibility</string>
<string>NeverVisible</string>
<string>NeverVisible</string> to:
<string>AlwaysVisible</string>.
Then quit TextEdit (it auto-saves the file now) and relaunch the Finder:
killall Finder
You should notice your Computer's name in the Favorites section of the sidebar.
Finally hold command and drag your Computer name to the top of the Favorites list.
[crarko adds: As usual, be sure you have a backup before making these changes. TextEdit will let you save the old version prior to editing.]

