How can I install a Windows 7 into BootCamp via USB

Through Disk Utility you can restore the Windows 7 ISO onto the flash drive.
When you hold option at startup to access the boot options the disc will appear.

Note: When you do the restore through Disk Utlity make sure your flash drive is partitioned correctly.

It needs to be partitioned as Mac OS X journaled (HFS+) - this will enable the GUID partition map (so the boot menu can read the ISO). Next, click source and within the finder locate the ISO and you're set.

With Lion, Apple has added an option to BootCamp that will create a bootable USB Windows 7 install disk, this option is only enabled in Mac models which don't have an optical drive. To enable this option just follow this guide.

This worked for me:

Burn ISO to bootable USB flash drive in Mac OSX terminal (via command line Diskutil)

• Convert the .iso file to .img using the convert option of hdiutil (e.g., hdiutil convert -format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso)
• Note: OS X tends to put the .dmg ending on the output file automatically.
• Run diskutil list to get the current list of devices
• Insert your flash media
• Run diskutil list again and determine the device node assigned to your flash media (e.g. /dev/disk2)
• Run diskutil unmountDisk /dev/diskN (replace N with the disk number from the last command; in the previous example, N would be 2)
• Execute sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m (replace N with your disk number)
• Run diskutil eject /dev/diskN and remove your flash media when the command completes

Make sure to run the Bootcamp assistant before you try to boot off the flash drive. Follow the instructions in How to install Windows 7 from USB, then see if the computer will recognize the flash drive when you boot up. To choose different boot media when a Mac starts, press Option, then select the flash drive if it appears.

If that doesn't work, post in the comments; there may be more steps required to get it to boot off of the external drive.