I can only get a mono signal when recording with a USB device in Windows Vista or Windows 7.
Windows Vista and Windows 7 sometimes seem to default to mono when recording from a USB device. To fix this, right-click on the small volume control icon in your Windows taskbar and select "Recording Devices." Next, right-click on the USB recording device and select Properties. On the Advanced tab, change the Default Format setting from "1 channel..." to "2 channel, 16 bit, 44100 Hz (CD Quality)." Finally, click OK to save that setting. You should now be able to record in stereo from your USB device.