Apple is selling Mac OSX on a Thumb Drive, but you can save some bucks and roll your own as simple as this:
1. Preparation: USB Stick
Insert an USB Stick that is big enough to fit the whole Mac OSX Lion installer. I can recommend the SanDisk Cruzer Blade 8GB, which you can get for less than $5.
2. Mac OSX Lion 10.7
get Mac OSX Lion from the App Store and download it.
3. Extracting the Install Data
Right click (option-click) on the just downloaded file and select "Show Package Contents". Get the file Contents/SharedSupport/InstallESD.dmg and copy it to your Desktop.
4. Disk Utility
Open "Disk Utility" and drag InstallESD.dmg from the desktop to the left sidebar. Select the attached USB from left side and click on “Partition” tab. Select "1 Partion" and from choose “Mac OS Extended (Journaled)” as partition type. Click on “Option” at the bottom, select “GUID Partition Table” and press OK -> Apply.
After formatting is done, select "Restore" and choose the USB Drive as destination and InstallESD.dmg as source. Click on "Restore" to have your Mac OSX Lion Boot drive created.