I don't use a dock, but here are some quotes from October's Stereophile Magazine iPod review. Wes Phillips is the author of the review:
..I ran the line-out from the docking
cradle (thus bypassing the iPod’s volume
control) to a Musical Fidelity Tri-Vista
300 integrated amplifier, which drove a
pair of Amphion Xenon floorstanding
loudspeakers....
..Best of all—and, to my ears,
completely indistinguishable from the
original CD—was AIFF. Dynamics were
impressive, imaging was nuanced and
detailed, and the frequency extremes
sounded extended and natural. On my
reference rig, I could listen with
immense pleasure for hours on end to
files ripped in AIFF....
..The iPod didn’t invert absolute
polarity,and the source impedance was a
suitably low 5.5 ohms over most of the
audioband, rising slightly to 15 ohms at
20Hz....
..the iPod’s behavior suggested good DAC
performance on these tests, with
linearity error remaining below 2dB down
to –110dBFS...
..At low frequencies and high levels,
the iPod’s distortion spectrum featured
primarily second and third harmonics,
with other harmonics below –90dB
(0.003%)....
..the presence of a strong component at
24.1kHz suggests that the iPod’s
reconstruction filter is relatively
“leaky.”...
..using the Miller Audio Research Jitter
Analyzer to look for word-clock
jitter-related spuriae in the iPod’s
analog output signal. The overall result
is superbly low, at 225 picoseconds
peak–peak....
..The iPod’s measured behavior is better
than many CD players...
..Excellent, cost effective audio
engineering from an unexpected
source....
