Ron Paul will be considered "serious" if he can come to the Republican National Convention with a meaningful number of delegates in his pocket....and the delegate race is close between two of the top contenders.
He
currently has two and the Republican party still has "winner take all" states (the Democrats do not).
Guiliani's strategy was to bypass the first few states, hope that different candidates win in those states (exactly what transpired), and focus his attention and resources on winning Florida and all of its 57 delegates to jump start his campaign going into Super Tuesday. We'll see if that was a good strategy or not.
Either way, if Pau's trend of getting 10% or less of the vote continues, he wont be going to the convention with many delegates and his presence will have no impact....much to the chagrin of his supporters.