"Erase and install" is Apple's terminology for totally wiping the hard drive, then installing the operating system as is. The last time "clean install" was used by Apple was in Mac OS 9. Then it referred to renaming the System Folder, and creating a new one in its place. Between 10.2 and 10.5 Apple created a similar process known as Archive and Install, as well having an "Erase and install" and install option on the installer disc, as well as an Upgrade and install which just updated the operating system from what it was without changing the System folder.
Snow Leopard condensed it all to two different install options. The one on the installer simply replaced the System folder, with one functional for the operating system chosen. When downgrading this left behind applications too new for the operating system. An erase and install was documented in the support notes, as erasing from Disk Utility, then installing from the DVD the operating system.
Lion just has one option that we are aware of, and that's updating a 10.6.6 or 10.6.7 installed system. Until it is released it is unknown if imaging, or cloning to another hard drive will work as expected, or if it will work within licensing restrictions.