AdFreak put together a list of 30 recent redesigns, which is an interesting examination of this design issue.
To me, Burger King, Cisco, KFC and UPS got it right
AOL and Kraft... not so much.
Now here's a twist on the whole issue. The exception to prove the rule if you will. See what Sprint did? They completely abandoned their logo, but with good intention. They redesigned the logo in conjunction with a major rebranding effort, supported with extensive mass media. Remember the pin-drop ads? Me too. The logo was redesigned to support that re-branding. The lesson being: if you are overhauling your brand, you have permission to overhaul your logo in support of it.
But only if you have Sprint money to do so.