# get a list of your virtualbox virtual machines
[me@mtbox ~]$ vboxmanage list vms
"Windows SBS Server 2011 Trial" {d0c766e8-252b-4016-a37f-db60894230eb}
"LSMB" {259dea54-ef0a-48c2-b06c-fa164e87abab}
"debian-6.0.2.1-amd64" {91b7ca3-2162-470e-aadf-df46046a468c}

# start the selected one using the uuid or the name in quotes
[me@mtbox ~]$ vboxheadless -s 91b7ca3-2162-470e-aadf-df46046a468c
Oracle VM VirtualBox Headless Interface 4.0.12
(C) 2008-2011 Oracle Corporation
All rights reserved.

# power it off 
#(you need to be logged in as the same user running vboxheadless)
[me@mtbox ~]$ vboxmanage controlvm 91b7ca3-2162-470e-aadf-df46046a468c poweroff
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

Note: If you want to use VirtualBox.org's VRDP protocol (use remote desktop protocol to connect to a headless machine). You need to install the Extension Pack from virtualbox.org

[me@mtbox ~]$ vboxheadless -s 91b7ca3-2162-470e-aadf-df46046a468c
Oracle VM VirtualBox Headless Interface 4.1.0
(C) 2008-2011 Oracle Corporation
All rights reserved.

VRDE server is listening on port 3389.

Sources
[1] http://www.virtualbox.org/manual/ch07.html
[2] http://www.virtualbox.org/wiki/Downloads
[3] http://www.virtualbox.org/wiki/Linux_Downloads