I'm still looking for a way of getting analogue input into my Raspberry Pi so I can build an ultrasonic "bat detector". I looked at this a few days ago, and came away disappointed with the slow speed of all the analogue input devices I could find.
I think I may have found an answer. I had almost forgotten I had it, but a few months ago I bought a "Freescale Freedom" development board from Farnell. It's a bit like an ARM equivalent to an Arduino, except that it is considerably faster, has more memory and has some cool built-in devices (three-colour LED, three axis accelerometer, capacitive touch "slider", etc.) Like the Arduino it also has on-board analogue inputs, but these seem to be much faster. This ADC tutorial seems to indicate that the board can be set to do an analogue to digital conversion in as little as 1.2 microseconds, which equates to a sample rate of well over 800,000 samples/per second.
Best of all is its price. I bought mine for £8.30 - that's less than I paid for a real-time clock module for my Raspberry Pi!
Of course, such fast sampling now raises other issues. The freedom board has 16Kb of RAM, which is a lot for programs, but not much for data arriving at this rate. The Raspberry Pi has plenty of memory, so I would need to find some way of getting information out of the freedom board and into the Pi as fast as it arrives. And that is a whole new challenge...