As mentioned in this hint, server icons will appear at the same position you left them when they were mounted the last time. For me, this is more of a bug than a feature, so I added the following command to a login script to get the good old behaviour back:
defaults delete com.apple.finder FXDesktopVolumePositions
This deletes the server icon positions every time you log in to your account, so the icons will appear in the good old order: At the next empty place from top right. Use at your own risk.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071221053346852