Several news outlets, among which was the Washington Post, claimed that upon agreeing to install Internet Explorer 8, the new browser would automatically make itself the default one, without notifying the user with a notice or a confirmation dialog. That is of course totally unacceptable - had it actually been that way.
Apparently, users simply weren't paying attention. Many often scorn what we call regular home users for not paying any attention to warnings or dialogs, but as it turns out, the tech savvy are just as vulnerable to it. Take a look at this selection dialog that is presented during Internet Explorer 8's installation process:
As you can see, the fact that IE8 will be made the default is listed under "Express", and since none of the two options (Express or custom) are checked by default, affected users specifically chose the express option. If you then get bitten because you weren't paying attention... Well, that's why advanced computers user should never make fun of those that are not advanced.
Still, it's a slightly clumsy move by Microsoft, but to claim that Microsoft is using its monopoly position to manipulate the browser market (as Opera is stating) in this case seems a bit over the top. It vaguely resembles Apple, who tried to cheat Safari onto unsuspecting Windows users through its Software Update tool installed alongside iTunes and QuickTime for Windows. Microsoft's move is not nearly as sneaky, though.
Just pay attention to what you're agreeing to, and never make fun of normal users always pressing "ok" without thinking ever again. And it goes without saying that you shouldn't use IE anyway, as it has the world's most horrible browser interface.