Lion reinstall

I have an iMac and a Mackbook Pro. Both are less than one year old. Originally bought with Snow Leopard but updated to Lion last month.
I need to have the option to reinstall Lion from a local external copy just incase I run into problems someday. I have created a bootable 8GB USB pen with a copy of lion (from guides online).

However, here is the problem. Both my systems use full disk encryption (from within Lion). This means when I boot either machine and hold down the 'option key' to select the disk to boot from, it will only show me the encrypted internal disk! I do not get the option to boot from any USB drive connected. I'm sure I could decrypt the drive then this boot option would work, but I don't want to do that!

I use encrypted drives purely to protect my data if any numpty decides to steal my machine.

Any help appreciated.
Paul

In reply to a post by woooshuk:

I have an iMac and a Mackbook Pro. Both are less than one year old. Originally bought with Snow Leopard but updated to Lion last month.
I need to have the option to reinstall Lion from a local external copy just incase I run into problems someday. I have created a bootable 8GB USB pen with a copy of lion (from guides online).

I burnt the image to a DVD and have successfully wiped and reinstalled my MBP (bought in June) with this disc.

Having an encrypted partition (I assume with FileVault 2 ?) should not stop Lion Recovery from booting, as they are separate partitions.

You have tried powering off, holding down Command + R and turning on and keeping them held down until the Apple logo appears?

Or holding OPTION instead when powering on and you should get the boot prompt?