It appears that the Google Local Business Center is not working for many businesses interested in updating their Google Maps/Local business listing. A Google Maps Help thread (well, there are a few) reports the issues from several users.
In short, it seems like when some try to access their local business listing they are presented with the following error:
We're sorry, but we are unable to serve your request at this time. Please try back in a few minutes.
This has been plaguing some business owners for over three days. There is no word from Google on the issue yet.
Forum discussion at Google Maps Help.