Fair warning, downloaded Lion upgrade and iMac began to install. Then it just hung then failed and my machine shutdown. Powered back on
and system started up with Apple logo then turned itself off. Run Disc Utility and said HD was damaged and unrepairable. Tried boot from install disc and running Disc Utility but the outcome was the same.
If any of you encounter the same problem and you don't have a back-up like me (I know...i know!) - I did find a way to rescue everything. I hooked up my imac to another (Firewire) with 'Diskwarrior' on - held down Apple and 'T' on broken mac and was able to run Diskwarrior to at least let me recover my phtots and work.
So am currently re-installing all files and software updates. That'll teach me not to use Time Machine.
Apparently I'm not the only one this has happened to!