Google is apparently doing more punctuation indexing these days than it has in the past.
Honestly, I can't recall ever searching for just a "." or just a "!" or other punctuation marks, but evidently Google historically has not provided results for them. Now they do, as Alex Chitu at Google Operating System points out.
Here's a search for ".":
Here's one for "!":
And one for ",":
Bing does not return results for any of these.
Neither does Yahoo. Neither does Blekko. Neither does Facebook.
Other punctuation/symbols Google is showing results for include: :,;,#, %,@, ^, ), ~,|, ", <, and $.