(...)Dual integrated 10 bit per channel 400 MHz DACs
DAC means digital to analog converter. This is totally irrelevent unless you're using an analog VGA output.
16 bit per channel floating point HDR and 10 bit per channel DVI output
I'm not exactly sure what they mean here (probably just marketing BS), since DVI is limited to 8-bit; only HDMI 1.3 and displayport can handle 10-bit color. Maybe their chipset is capable of outputting 10-bit color in some hypothetical scenario, but this feature is not enabled on any actual shipping cards, certainly not under Windows Vista or earlier. And as far as I know not under Windows 7 either, unless they've released new drivers to enable this functionality (which they haven't, to my knowledge).
Programmable piecewise linear gamma correction, color correction, and color space conversion (10 bits per color) (...)
In other words, internal processing pipeline, just like I said.
If you think you're getting 10-bit color with an x1900 or any other Radeon card under Windows, show me the screen shot of the display settings dialog where it lets you select that option.