Not sure what G's intent is with this, but the end result will no doubt be monetization. That is, if they get away with it and not enough people cry foul.
Actually, some theatres do this and get away with it just fine. And have been doing it since the 50s (maybe before) - but there is a clear break between the movie and the ads. Ads at the begining, some theatres insert an "intermission" that inevitably has ads as well.
The issue with the G toolbar function is that when the feature is activated, that clear break between the published content and the advertising disappears. Or, at the very least, the line is blurred.
Worse yet, I truly fear for the "relevance" of the links they'll present. From looking at their SERPS lately, and the crummy ads that creep into my adsense from time to time (no matter how carefully I encode my pages to make it as clear as possible what they're about), G just hasn't been doing a great job on the "relevance" front lately.
And for all those sites that are simply not designed with search engines in mind, the overall relevance of the links is likely to be staggeringly poor.
There are many, many issues that need to be adressed before they take this to prime time. And about 10 years worth of technological maturation before it becomes truly feasible.