I need to hide the cursor when USB mouse is disconnected from the system, and show it when the mouse is connected. Any ideas how it can be done?
Qt itself does not provide such a module. There is a user-made class called QDeviceWatcher, you could give it a try.
If you are using QtEmbedded Linux, you have a folder with all USB devices info (vendor id, etc), probably in /proc/scsi/usb-storage file. So you probably could use QFileSystemWatcher to handle modification in this folder.
See: Qt Centre Post for more detail.