It's occured to me that this isn't actually unprecedented - and that it was met with huge disfavour on previous attempts by other companies too.
Adware was tolerated only till people realised it wasn't normal to be bombarded with unsolicited avertising, and got software to remove it.
Then there was Verisign with their unregistered domain hijacking last year or the year before - every domain people entered that didn't exist went to a page they advertised on. That was shut down days after it launched.
Funnily enough, Microsoft tried to get into the game too, and developed software or plans for software that would scan web pages you're viewing and change key words to links to advertisements. And that fell through too.
So, historically speaking (in terms of the internet) this is not the first time a company has tried to hijack content or peoples experiences. And really, gmail is just email hijacking.
Google's "don't be evil" became a moot point when they floated. They have a legal obligation to their shareholders to make money, and to make more of it. Their hijacking of emails is purely about money.
The sad thing really is the businesses who will suffer in the interim period (while Google profits) until Google are simply required (whether by public pressure or courts) to remove targeted advertising from email messages.
It may seem from my arguments that I'm against Google and I'd like to clarify that I'm not - I use Google every day (just not gmail), and I like it.
But I have no sympathy for their decision to push their advertising revenue even further under the guise of providing a better (or new?!???) service for the public, while doing or risking irreparable damage to business/client relations before they even get a chance to form.
Aside from that, Gmail really doesn't offer anything new to consumers. It's been engineered from day 1 purely as a way to deliver more advertisements and generate more money. Marketing hype over the "search your email / 1gb of email" have made public perception regard it as far more than it really is, which is a throw-away expense dwarfed by the profit of advertising on every email that hits its servers.