Internet Explorer 9's use of application reputation to warn users that they're installing software which hasn't been widely tested is a familiar tactic, albeit one that's previously been the domain of security software. If you're regularly testing software that changes frequently (such as the Chromium project on which Chrome is based), that tactic could becoming annoying fairly quickly.
Google Operating System outlines how the process of trying to install Chromium builds — files which of necessity change frequently — results in IE9 popping up four separate warnings before you can actually run the installer, and requires you to work through some not-very-intuitive menu options to make that happen. It's well-known that many people click through warning dialogues, so that might act as a delaying tactic, but I'm not entirely sure that people will be any more attentive on the fourth screen than they were on the first.
I wouldn't want to overstate this as a major issue: anyone regularly testing Chromium builds isn't likely to be using IE9 as their main browser, for starters. And for the mass market users who represent the main IE audience, having a warning system in place that stops malware being installed is not a bad idea. Microsoft's own claim is that this won't happen for the average user more than two or three times a year, However, for more advanced users tempted by IE's improved performance profile and stripped-down looks, it is another factor to bear in mind if you're considering the switch but frequently work in test or development projects.
Installing an Application Using Internet Explorer 9 [Google Operating System]