Paul Gielens has a thought-provoking posting on 'Agile Does Not Scale Very Well'. I posted a response to it but here it is again -
Having worked primarily for the Fortune 100 companies, I have seen it first hand that Agile (or any other "loose" methodology) won't work. Large organizations need a very regimented methodology (process/procedures) for it to function. Moreover, large organizations have very compartmentalized process for 'compensation', i.e., who contributes to development/projects and how so that they may get the rewards/credits.
Agile was never meant to be for large-scale organizations - it'll need a very serious change in management structure and thinking for it to happen.
About a year ago, I had posted something similar to the above at David Anderson's blog (MSFT employee who works on Agile Projects - http://www.agilemanagement.net/) but haven't heard anything about it (yet).