It seems that the answer is "yes", judging by the number of my colleagues who are throwing the idea at me. It makes sense: several trends are working towards it.
First, everyone is now gathering around the same integration point: digital + other disciplines (whatever they are).
It works both ways: ATL and BTL agencies needing to prove digital credentials and digital agencies needing to prove their branding and DM expertise.
More and more of digital agencies are exercising strategic (and even creative) lead on integrated, multi-agency projects.
Finally, digital agencies are getting bigger as well, which means more acquisition muscle.
I'm waiting for the first big example, probably a big digital independent buying a smaller DM outfit. Don't say you haven't been warned.