OS X 10.7 Lion won't be available on USB until August -- and then you've got to shell out about $70 for it. Why wait? Just create a Lion-bootable USB drive yourself. That'll not only save you some money. You'll also get some piece of mind in case of a system crash.
The optimum time to make your bootable disk is early in the install process. Otherwise, you won't be able to get the DMG file you need without having to download OS X Lion all over again. And that's too time-consuming.
If you did already install it, then follow Apple's Lion Recovery instructions to re-download OS X Lion again.
Either way, as soon as you install, get ready. This is key. The install starts automatically -- you need to stop it. At the initial screen asking if you want to press Continue, DO NOT CLICK CONTINUE!
Don't hit Command[Q] either. Just leave the screen up or minimize it, and follow these instructions carefully.
Locate the installer in your Application folder and click it once to select it. Press [COMMAND][D] to make a duplicate copy.
This will create a copy of the installer labeled Install Mac OS X Lion Copy. Use it to recover from if you make a mistake in this How to.
Right-click and select the option to Show Package Contents.
Double-click the Contents folder to open it.
Inside the Contents folder you will find a SharedSupport folder. Double-click it to open that.
Here you'll find the InstallESD.dmg inside the ShareSupport folder. If you backed up the installation file as instructed and don't see it here, find the backup.
Apple, which is selling a $70 USB install disk of its own in August, destroys this file at the completion of most installations.
Right-click the InstallESD.dmg file and hit Copy. Switch to Finder and paste the file to your Desktop.
Note: Paste this -- do not drag it. Doing so endangers the installation application.
Open the Applications folder in Finder and locate the Utilities folder. Double-click to open it.
Double-click the Disk Utility icon to open it.
Move the ESDInstall.dmg file from your Finder desktop to Disk Utility, as shown below. Drop it into the left-most windowpane inside of Disk Utility.
In Disk Utility, click the InstallESD.dmg icon.
Click on the Restore tab. Now insert your blank USB drive. Once it appears on the Finder desktop, move its icon to the Disk Utility window. Drop it onto the Destination.
If you are prompted for your user ID and password, enter them and hit OK. This starts the copying process.
The process copies the InstallESD.dmg image to the USB stick, verifies it and completes. Now just wait for your new OS X Lion install USB stick to finish. You're done.
Keep the drive in a safe place. You never know when you'll need to do a fresh install. Use this disk to boot any OS X 10.7 Lion compatible Mac. Install OS X Lion 10.7 just as you installed previous versions of OS X. Nice job.