According to a Google Groups post, Maps Guide Brian has informed us that users can now write reviews for businesses in eleven countries. The catch: you need a Google account to do so.
To write a review for a business, just search for it, expand the information window by clicking on the "+" sign, and click on "Write a review" at the top. You can also expand the window by clicking on "more info" in the left panel. Once you submit your review, it'll appear on Google Maps right away.
Barry experimented with this feature and tried it out on our company, as you can see in his Search Engine Land post on June 19th. He walked us through the process of creating a review, and I just checked -- it's still there on June 26:
Cool stuff. I guess the reputation management police might be after the real negative reviews, but positive reviews won't get hit so hard.
Forum discussion at Google Groups.