Apple has moved Directory Utility from /Applications/Utilities/ to /System/Library/CoreServices/ .
It can also be accessed through the Accounts System Preference by going to Login Options and clicking "Edit" next to the network account server at the bottom. There is an "Open Directory Utility…" button there.
It also appears that Firmware updates have a new location in a folder in CoreServices, which is rather nice.
Next up, it looks as though the Kerberos application has been renamed "Ticket Viewer".
The 10.5 only application "Directory" seems to be missing in action. I was hoping that its functionality was merged into Address Book, but it does not appear to be the case.
Please add any others in the comments below.