You might find yourself looking for "net send" in Vista and wonder where it went?  Before you flip out, there is a solution.  Use MSG.exe

C:\Windows\system32>msg /?
Send a message to a user.

MSG {username | sessionname | sessionid | @filename | *}
    [/SERVER:servername] [/TIME:seconds] [/V] [/W] [message]

  username            Identifies the specified username.
  sessionname         The name of the session.
  sessionid           The ID of the session.
  @filename           Identifies a file containing a list of usernames,
                      sessionnames, and sessionids to send the message to.
  *                   Send message to all sessions on specified server.
  /SERVER:servername  server to contact (default is current).
  /TIME:seconds       Time delay to wait for receiver to acknowledge msg.
  /V                  Display information about actions being performed.
  /W                  Wait for response from user, useful with /V.
  message             Message to send.  If none specified, prompts for it
                      or reads from stdin.

C:\msg /server:brad-dc-01 console "You're machine needs to be rebooted."