Yahoo Unveils Mobile Search, Google Rivalry Rages  SAN FRANCISCO (Reuters) - Internet media company Yahoo Inc.  &lt;A HREF="http://www.reuters.co.uk/financeQuoteLookup.jhtml?ticker=YHOO.O qtype=sym infotype=info qcat=news"&gt;YHOO.O&lt;/A&gt; on Wednesday launched its Web search service aimed at  mobile phone users in the United States, opening a new front in  its rivalry with Internet search leader Google Inc. &lt;A HREF="http://www.reuters.co.uk/financeQuoteLookup.jhtml?ticker=GOOG.O qtype=sym infotype=info qcat=news"&gt;GOOG.O&lt;/A&gt;