Hi. I just acquired an Arduino LilyPad USB today, and had trouble making it work on my GNU/Linux computer (i'm using, and loving Ubuntu). The problem was that Arduino IDE was telling me that it couldn't find a Leonardo device. I'm really sorry i haven't got the exact error message anymore, it would be great for people who search the forums. Sometimes i would get a message 'Serial port /dev/ttyACM0 not found' instead.
Anyways, after some headscratching and debugging, i found out that a component of the Ubuntu Network Manager called modemmanager was snatching the /dev/ttyACM0 device. I filed a bug report on Ubuntu Launchpad #1153632, made a bugfix, and submitted a merge request to Ubuntu.
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So it should be fixed for everyone who upgrades their Ubuntu eventually when 13.04 is out, or if this bugfix get's backported. Meanwhile, if you are using an Arduino LilyPad USB, create a file called /etc/udev/rules.d/70-arduino-lilypad-usb.rules with the following contents:
# Arduino LilyPad USB
ATTRS{idVendor}=="1b4f", ATTRS{idProduct}=="9207", ENV{ID_MM_DEVICE_IGNORE}="1"
ATTRS{idVendor}=="1b4f", ATTRS{idProduct}=="9208", ENV{ID_MM_DEVICE_IGNORE}="1"
And then run sudo service network-manager restart or restart your computer.
I hope this helps, and i hope people will find here via Google, and i also encourage you to see the Ubuntu Launchpad bugreport Bug #1153632 “Arduino LilyPad USB is wrongly grabbed by modemman...” : Bugs : modemmanager package : Ubuntu