A Google Groups thread has advice from Maps Guide Jen about how to fix map errors.
Jen explains that "map data errors - such as misspellings of town names, missing streets, etc - are often not the same as your business being located incorrectly." To suggest fixes for town names, missing streets, etc - Google uses both NAVTEQ and TeleAtlas for these things. So if you report the issues to them, Google will update the maps when they are released to them.
What if there are other issues?
Google has a Google Maps Contact Form where you can report issues with Search Quality, Feedback and Suggestions, How do I?, Report a problem, I'm a Business Owner, and Maps API Feedback.
Forum discussion at Google Groups.