I have done a bit of experimenting with the general purpose clocks.
This information appears to be correct (on the post date).
Refer to http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf pages 102-108. The clock frequencies were determined by experiment. The oscillator (19.2 MHz) and PLLD (500 MHz) are unlikely to change.
0 0 Hz Ground
1 19.2 MHz oscillator
2 0 Hz testdebug0
3 0 Hz testdebug1
4 0 Hz PLLA
5 1000 MHz PLLC (changes with overclock settings)
6 500 MHz PLLD
7 216 MHz HDMI auxiliary
8-15 0 Hz Ground
The integer divider may be 2-4095.
The fractional divider may be 0-4095.
There is (probably) no 25MHz cap for using non-zero mash values.
There are three general purpose clocks.
The clocks are named GPCLK0, GPCLK1, and GPCLK2.
The clocks are accessible from the following gpios.
Don't use GPCLK1 (it's probably used for the Ethernet clock).
gpio4 GPCLK0 ALT0
gpio5 GPCLK1 ALT0 B+ and compute module only (reserved for system use)
gpio6 GPCLK2 ALT0 B+ and compute module only
gpio20 GPCLK0 ALT5 B+ and compute module only
gpio21 GPCLK1 ALT5 Not available on Rev.2 B (reserved for system use)
gpio32 GPCLK0 ALT0 Compute module only
gpio34 GPCLK0 ALT0 Compute module only
gpio42 GPCLK1 ALT0 Compute module only (reserved for system use)
gpio43 GPCLK2 ALT0 Compute module only
gpio44 GPCLK1 ALT0 Compute module only (reserved for system use)