Google has been promoting tags for Google Places to businesses for the past few months or so. Google said, "adding a tag to your Google Places listing allows you to highlight info that you want your customers to see." But Google is no longer allowing you to highlight your website URL, I guess because your customers don't want to see it (mocking Google here).
A Google Places Help thread has one complaint where Googler Brianna replied:
We're constantly refining our tag types any website was a previously offered tag type. We no longer offer this option as of today, 10/25/2010.
As we think about building new tag types, we're interested in what appeals to you about the website option, so please feel free to reply to this post. We will consider all of this feedback in our thinking.
We still hope you'll find another tag type from the ones currently offered that works for you!
Local search expert, Mike Blumenthal said the website URL tag option was "the most effective" tag available and he is sad to see it go.
Those who have the URL option already in place are able to continue to use it, but you can't change it, if you do, it will be lost. So those who are grandfathered in, stick with it - until Google snatches it away.
On a side note, it seems like some people are having issues with duplicate tags since yesterday.
Forum discussion at Google Places Help.