John Battelle links to an MBA paper by Julien Decot and Steve Lee. From the paper:
(the) main takeaway from our analysis: assuming the profit margins from their current businesses remain similar to their current level, or worse, if they face increased margin pressure, it is pretty likely that eBay will consider seriously entering the online advertising market at this point….
…Based on our high level projections, we can foresee a mismatch between demand and supply
that could possibly induce some price pressure on Googles core ad market. Moreover, in order to
reach revenue levels built into Googles valuation, Google will have to enter markets outside of online ads.