Santorum wasn't runner up. Ron Paul was and where's our speaking slot? Snubbing Paul would be worse Phil. Rick forfeited second place when he withdrew while Ron Paul stayed in the race.
Simply remaining in the race well past you have any chance of winning does not make you the runner-up. Santorum won over 21% of the popular vote, to Ron Paul's 11%. Santorum won 10% of the delegates to Ron Paul's 4%. By almost any measure, Santorum outperformed Ron Paul. I would think that certainly puts him in the runner-up position.
Now whether he should speak or not is another question. He's divisive, and much like Palin, while party faithful may like him, he could be a turn-off to the general electorate.