Each Gumstix expansion board that has a "3.5-mm stereo headset jack", such as the Gallop43 board, has a mic-in line so a microphone can be plugged in directly. Make sure to use a kernel that is more recent than 25 Sept. 2010 - see here for recent pre-built images.
Each expansion board that has "stereo audio in" rather than a "headset jack", such as the Tobi board or Palo43 board, does not have a mic-in line so a microphone cannot be plugged in directly. An amplifier would be needed for a microphone since there isn't a pre-amplifier.
Check the features listed on the product page of each expansion board to determine if the expansion board has a stereo headset jack or stereo audio-in.
Capture Interface Settings
The audio Line-in uses the 'Analog Right AUXR' and the 'Analog Left AUXL' lines; the Mic-in uses the 'Analog Left Main Mic' and the 'Analog Right Sub Mic' lines. Use amixer or alsamixer to select the correct capture interface for your application.
Currently, Line-in is selected as the default capture interface. You could switch to the Mic-in interface like this:
$ amixer -c 0 sset 'Analog Right AUXR' nocap $ amixer -c 0 sset 'Analog Left AUXL' nocap $ amixer -c 0 sset 'Analog Left Main Mic' cap $ amixer -c 0 sset 'Analog Right Sub Mic' cap
To test, you might use something like this; it records for two seconds and then plays back the recorded sound.
$ arecord -d 2 -f cd -t wav foobar.wav;aplay foobar.wav