Works in Ubuntu 12.04.
For those of you using Linux, the .techniclauncher folder is in your home folder and if you are using a GUI then you need to "Show hidden files" to see it. Follow the instructions at the start of this post TO THE LETTER but obviously make adjustments based on your linux version.
Alternatively you can type this in a terminal once you have downloaded the specified file. For non Ubuntu users, your download directory may be different.
mv ~/Downloads/minecraft.jar ~/.techniclauncher/cache/minecraft_1.2.5.jar
