Google (GOOG) will derive 20% of its clicks on paid search ads in the U.S. from consumers using tablets this year, up from 10.7% in December, says a report released Tuesday by Marin Software. Google already is the No. 1 seller of paid search ads. Global tablet unit sales rose 75% in Q4 vs. Q4 2011, says market tracker IDC. Marin's report also found that conversion rates on ads on tablets rose 31% in 2012 vs. 2011, far ahead of conversion rates on