No WLAN without Ethernet

I have installed Ubuntu Precise 12.04, and have discovered a weird problem with the network interfaces.
What I have setup is a USB wifi adaptor connected to my Beaglebone Black, and sometimes I connect it to Ethernet for debug. The wifi is not exactly used for connecting to a network, but a script I have running that utilises wifi doesn't work without Ethernet being connected.
What happens is on boot with no Ethernet and just wifi, it will sort of hang (but not completely the LEDs still flash) for a few minutes, then start up wifi.
Interestingly, I had the same problem with my Raspberry Pi, so I don't think this is a specific issue with just my setup.
So basically what is happening is the beaglebone black refuses to even activate WLAN without Ethernet connected as well.
Could anyone help me out?