Interesting question on the ripple. Probably most types of regulator will be ok, unless you're trying for good quality sound from the analog output
. I'm guessing as much as 200 mV peak-peak of ripple/noise is ok if you are getting 5.0V to the board. If it's only 4.75 V on the board, that amount of noise might be more of a problem. My adaptors typically have only a few mV of ripple and I haven't tested susceptability to that.
As you see in the RPi schematic http://www.raspberrypi.org/wp-content/u ... s-R1.0.pdf
the big 220 uF capacitor (C6) is directly on the +5V0 rail, so you have that helping you regardless how you connect the power. However, if you go the intended route through the microUSB filter, that input polyfuse F3 has some resistance which helps you with noise, forming an RC filter in combination with C6, C3 and C2. If the F3 resistance is 0.3 ohms, that gives you a low-pass filter with a 2.4 kHz shoulder. So it helps cut down 100 kHz switcher noise, but is not any help with 100 or 120 Hz ripple.