As much as people seem to be laying most of the blame at NBC's doorstep, people shouldn't forget that it was JAY LENO who announced he was leaving the Tonight Show several years ago, and only after that announcement did NBC sign Conan O'Brien to replace him.
Jay then changed HIS mind, but NBC couldn't really do anything about it since Conan was Tonight's new host (start date and all). So, NBC gave Leno his now failed variety show at 10PM.
From Late Night with David Letterman (1/14/10):
Dave's Monologue - Part 1:THIS MESS IS ALL JAY LENO'S DOING (and NBC for enabling him). But that isn't stopping NBC execs from trying to pin the blame on Conan.
Dave's Monologue - Part 2 (Dave comes back to NBC/Jay after a few digs at Mark McGuire & Sarah Palin):
But, Dave isn't the only late night host blaming Jay. Jimmy Kimmel has left scars on the self-absorbed Leno too.
Anyway, it looks like Conan will be leaving NBC with a very large payout.