When you install OS X Lion, it creates a hidden Recovery HD volume on your Mac's startup drive. The Recovery HD contains everything necessary to boot your Mac and repair a hard drive or reinstall the OS.
The problem is that you have no choice about where the Recovery HD partition will be created; Lion puts it on the startup drive.
I have a problem with that. First, it's downright foolish to put the Recovery HD on the same drive you routinely boot from. If the drive develops a hardware-based problem, chances are the Recovery HD partition won't be usable. Would you back up your data to the same hard drive the data is on? Of course not. So why does Apple think it's okay to put the Recovery HD on the startup drive?
Fortunately, there's an easy way to copy the Recovery HD to another internal drive, an external drive, or a removable USB flash drive. I keep one copy on a USB flash drive and another on an external hard drive. This way, the Recovery HD is always available, plus I have a portable version I can carry around with me.
I will outline two methods for copying the Recovery HD to a new location. The first uses Apple's Recovery Disk Assistant, which can copy the Recovery HD to any external device. The second method lets you copy the Recovery HD to any device: internal, external, whatever. The first method, using Apple's utility, is the easiest; the second is the most versatile for more complex Mac installations.