What happens when you try? Error messages? Apple says you can:
iCloud automatically backs up the most important data on your iOS 5 device. Once you have enabled Backup on your iPhone, iPad, or iPod touch in Settings > iCloud > Backup & Storage, it will run on a daily basis as long as your device:
- is connected to Internet over Wi-Fi
- is connected to a power source
- has the screen locked
You can also back up manually whenever your device is connected to Internet over Wi-Fi by choosing Back Up Nowfrom Settings > iCloud > Storage & Backup.