How is it being closed source a meaningful distinction?
It does not matter what the end user can do, antitrust focuses on what the alleged monopolist is doing.
Also, choice is not a defense to antitrust under the Sherman Act. That's a common misconception among non-lawyers. Choice is only weak evidence, at best, that the an alleged monopolist's activities are not anti-competitive.
Chromium may be open source, but Chrome is not.
Excepting of course external plugin type issues (printing and Flash don't work yet), the browser runs super-fast, never crashes, and looks great. Some of the configuration options aren't complete, but those are minor issues (oh and I see they have added many of them).
Text rendering used to be horrible but it has gotten better. But if I can't even configure proxy settings without hacky gconf editing, that tells you that you are definitely using a browser in catch-up mode.