There are several features in OS X 10.7 that make a bootable backup disk somewhat redundant including:
  1. The bootable "Recovery HD" that is installed on every Lion boot disk (Reboot while holding down command+R)
  2. Time Machine
  3. The Apple App Store

You can boot from the Recovery HD as mentioned and do a full and complete restore of your system from any given hour, day, week, or month since you activated the Time Machine backups or you can go to the App Store and re-download and install OS X — which will be the current version at this unique point in time. This, of course, will not help in the event of a mechanical failure of your hard drive.

I suspect it is for that reason that some users feel uncomfortable without a bootable hard copy of their OS. In that case I can suggest three options in roughly the order of their popularity:
  1. A clone of your entire system on an external hard drive using Carbon Copy Cloner or SuperDuper. Both can be used free in their simplest version which is really all you need. The merits of both have been discussed at length but IMHO it mostly boil down to which user interface you prefer.
  2. Keeping a bootable copy of Lion on an external hard drive along with one or more volume testing and recovery apps such as Drive Genius, TechTool Pro, or Diskwarrior. Each of these cost around $90 but most will tell you having one or more of these is essential due to the very weak capabilities of Disk Utility. You would still need the Time Machine backup for recovery.
  3. A bootable recovery DVD. For instructions on doing that see this Apple Knowledge Base article. You would still want to have the Time Machine backup to recover your personal data files, settings, etc.

FWIW, I have a Time Machine backup and options 1, 2, and 3 but if I had to limit it to a single option it would probably be option 2. 😄

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein