Put Lion "Recovery HD" on USB
The best answer is one supported by Apple.

Why not use the new Lion Recovery Disk Assistant to make your USB bootable media.

The only requirements are listed as:


A Mac running OS X Lion with an existing Recovery HD
An external USB hard drive or thumb drive with at least 1GB of free space


Detailed, step by step instructions are updated at http://support.apple.com/kb/HT4848