Apple have released a Lion Recovery Disc Assistant for external drives so that these drives can be used to help should your internal drive develop serious problems.
The article explains it and offers links to the software.
What follows is for other readers who may be a bit confused.
The Recovery Disc Assistant offers the same services as the (invisible partition) recovery disc which is automatically installed on your drive when you install Lion, and available by restarting and holding down the Option key in the normal way to reveal startup options which can be chosen with the arrow keys or mouse.
You can reach the same result by following the instructions posted about halfway down "The Lion Roars" discussion (link repeated here)
Remember that installing it on a USB stick, a partition etc. will wipe that stick or partition clean. So make sure that you have a clean 8Gb+ stick or partition etc to install it on -- if you want it.
So: it's there as an alternative for when your normal Lion partition or drive won't mount, and your (invisible partition) recovery disc is unavailable due to a failure of some kind. It doesn't contain the Lion Installer, just the recovery tools and links to the appstore via it's own copy of Safari.
Five points:
1. This assistant is basically an un-frightening way to do the same thing as in the second paragraph above.
2. when you install Lion for the first time, make sure you have a backup copy of "Install Mac OS X Lion" on another drive, partition, memory stick, DVD etc. to avoid having to download it again.
3. it's always good to have a clone of your normal OSX drive or partition so you can get back to enjoying your Mac after an emergency with minimum fuss.
4. It's always good, if there is enough space, to have more than one partition on your Mac's drive with an alternative MacOS installation.
5. Time Machine doesn't reinstall the OS, just your network and files.
Hope this helps someone.
Euan Williams 