The internal microphone does not work after a default install. Append the line "options snd-hda-intel model=dell-s14" to file /etc/modprobe.d/alsa-base.conf, reboot and adjust the mixer levels.
Here is the fairly well done test results for a Dell E6410 Laptop running Ubuntu 10.04 LTS. I've seen all of the issues reported so far on my laptop and can confirm that the workarounds do actually work.