It used to be that BigCorps didn't want to rock the boat for fear of disrupting their businesses. These tech companies were built on the missed opportunities of the entrenched players, and with our current financial climate and its easy access to cash, they are not likely to make the same mistake?
For the most part, foundational technical innovation has always spewed primarily from entities large enough to bankroll it without any immediate or known ROI (universities, government labs, Xerox PARC, Sun Labs, IBM Research, etc).
The innovation from startups tend to piggyback off of that technical innovation. But the startups find unanticipated market applications, innovative business models, or incremental improvements/refinements for specific industries.
I think the biggest difference now is that modern BigCorps acknowledge those strengths of startups rather than dismissing them. And between the current financial climate and that acknowledgement, they're far more willing to buy up a startup that looks promising than they used to be.