I would like to report that I was able to get the Alfa AWUS051NH USB wireless adapter to work on a vanilla BackTrack 4 R2 installation on VirtualBox 4.0.4. I used the default settings/options for the entire process (VM creation, OS install, etc.), and the only "extra" steps I took were to run 'apt-get update && apt-get upgrade' and to install the VirtualBox Guest Additions (the VirtualBox equivalent of VMware Tools). I then plugged the USB adapter into the host, navigated to the VM window's menu, clicked "Devices" -> "USB Devices" -> "Ralink 802.11 n WLAN ". At this point, the AWUS051NH will be "plugged into" the virtual machine, and there should be relevant messages posted in /var/log/messages and a new interface should appear named 'wlan0' (verify with 'ifconfig -a'). Next, I enabled monitor mode on the device by running 'airmon-ng start wlan0', and the script successfully detected the chipset which it displays as "Ralink RT2870/3070" and uses the "rt2800usb" driver by default. Now that the USB adapter is in monitor mode (on 'mon0'), it's time to test packet injection with aireplay-ng. I ran 'aireplay-ng -9 mon0' and injection was successful.
I attached screenshots of all of the relevant parts to this post. Note that I did censor out the MAC addresses and ESSIDs to protect the privacy of my neighbors.
To the BackTrack developers: It would be appreciated if you could move the AWUS051NH from the list of "NON working cards" to the list of "Tested and working cards" on this page of the wiki: Wireless Drivers - BackTrack Linux