I would not say that ultrasound is highly directional, it just depends on the transducer.
Yes, you can use it for wireless telecommunication. In fact, the first remote controls for TV's were employing ultrasound. However, they were quickly replaced by IR remotes, for a few reasons:
ultrasound disturbed pets, though humans cannot hear them
the doppler effect can play tricks on you if you move the remote while sending a command.
Since they were using multitones (one tone for each command) and filters to separate the commands, the number of commands encoded could not be very large.
You could hardly use two remotes for two different sets, say for a VCR and a TV, since interference was almost guranteed
With IR, all the above disadvantages disappear. They use encoding, which increases the number of available commands, removes interference, etc.
I suggest you go with IR, it is easier to find transducers, too. An old TV or VCR will provide one. You can buy an old remote control just about anywhere. You will also find easily info on how to decode the commands.
Basically, using a microcontroller, an IR receiver and your TV remote control you can start DOING something in no time.