They say it best, so straight from the horse's mouth:
Ad Manager currently addresses the ad management and serving needs of publishers with smaller sales teams and effectively complements the DoubleClick Revenue Center, which is focused on publishers with large sales teams. We're excited to add DART for Publishers to our suite of products,and we're committed to the continued development and enhancement of DoubleClick's offerings.So basically all of the internet marketing industry's speculation earlier this week about what this means has been swiftly answered. Sort of.
It's still not clear the fate of affiliate network Performics, a DoubleClick company, and whether they'll stick around or if it'll be eventually swallowed. Personally I think it would be silly for them to absorb Performics. Instead they should pretty much do what DoubleClick did when they initially bought Peformics - just append the name.
Coming to an internet near you - Google Performics?