Anyway, I quickly tired of opening the Virtualbox Application just to launch the machine I needed (it's nearly always the same one even though I have many). So, I poked around in Google and found this great bit of instruction from Mark Bockenstedt.
Basically, you just have to create a script in Script Editor and save it as an Application. Then change the icon (if you want) and drag it to the dock. Here is all the code for the script.
do shell script "vboxmanage startvm Ubuntu"
Very simple, very easy.