Because Lion doesn't come on a pressed CD and is downloaded through the Apple App Store, it's a bit harder to swallow a catastrophic failure of your HDD considering you have no disk to put in and re-install. This is where Apple's Lion Recovery Disk Assistant comes in handy. It'll duplicate the OS's recovery partition onto an external drive of your choice. With that recovery disk created, you can then boot directly into an installer which will re-download the latest OS.
Of course, you'll require a disk larger than 1GB, a machine running Lion and the 1MB assistant which you can grab here.