After upgrading Dell Vostro 1014 laptop to Ubuntu 10.04 Lucid Lynx I found the old problem of headphones failing to mute the internal laptop speakers when plugged remains in the new release as well.
Three months ago I posted a workaround for Ubuntu Jaunty users in this post ALSA Jack Sense problem in Jaunty. However this solution doesn't work in Ubuntu Lucid.
To solve this issue you need to install linux-backports-modules-alsa-lucid-generic package and reboot your computer.
sudo apt-get update sudo apt-get install linux-backports-modules-alsa-lucid-generic
Now speakers should mute as soon you plug in your headphones. If that doesn't work try to pass the correct model name by adding the line to /etc/modprobe.d/alsa-base.conf file.
options snd-hda-intel model=dell-vostro enable=1 index=0