It may well be speculation VT, but it seems to me a useful strategy for Adwords advertisers if they want specifically to appear on certain sites, not only if they want a precense on a site that has a separate agreement with their competition. I don't really see anything unethical about it, but a reason why adsense publishers review the ads appearing if they may conflict with their own interests or those of their other advertisers.
In answer to your question, no i havent seen it specifically but i think this practice may be inevitable, as long as adsense spidering remains in it's present form, or of its very easy to "predict" the terms the mediapartners bot uses for certain pages or sites. When we had "similar sites" in the adnsense block they could more easily be targeted just by reading the bottom bar "other pages about xxxx" etc. That wouldnt be 100% as it probably uses the google search algo rather than the adsense bot data, but would give a better clue than anything else on what keywords you should bid for in Adwords to have a good chance of appearing on that page.