If you use one of the waterproof ultrasonic receiver/transmitter pairs and really drive the transmitter, by that I mean don't mess around with 5V, use an amplifier chip, 2 LM380's in a bridge configuration or an STK module. Those sealed ultrasonic transmitters will produce a >105dB and take a maximum input voltage of 140V! That is enough to hurt, so don't put your ears anywhere near it when it's operating, you wont hear it, but believe me you will feel it, like somebody sticking a knitting needle in your ear. Here in Australia you can buy a kangaroo scaring device that mounts on your front car bumper, I've seen 'roo's react to them at distances of over 100M! Again it's not something you can safely stick your ear in front of, it really hurts! Same goes for ultrasonic "intruder" detectors, some of those have a VERY high output.
Bat's, rat's dolphin's and whales have been "imaging" like this for ever! Dolphin's can stun fish with a concentrated pulse of sound.
I've been experimenting with a continuous output ultrasonic system. Rather than a pulse I've been trying to "illuminate" the area in front of a small rover, then using multiple receivers to "watch" the reflected sound. With 3 receivers I "fill" in the gaps between them by summing the outputs of the outer receivers with the centre one, sort of like synthetic aperture radar. I started out just watching the output on my CRO, as soon as I got my Mega and XBee modules I started visualising the A/D converters output as simple vertical bar graphs. You can drive around quite well without running into stuff.
Another thing I've tried is a few different shaped "reflectors" for the transmitter and receivers. A simple plate at 45 degrees above a horizontally mounted transmitter widens the field of the beam produced, as well as flattening it out. Building small horns for the receivers also is something to try.
You can definitely tell the difference between various materials and textures, grass has a different texture to gravel, bark and wood return a different sound texture to concrete, which is different to bricks.