A reader goes with the former:

I think Rush was dripping sarcasm throughout the first 30 minutes of his show.  It’s true that he started out sounding sincere, but he could not keep his vitriol at bay – he called the president “Osama” at least 5 times. And while it is just my opinion, I think his complimenting President Obama on coming up with the plan by himself was simply part of the narrative that Obama only speaks with the words I and Me.  Rush even provided a breakdown of the number of times the president used I and Me.

Another isn't sure:

In its transcribed form, this reads like sarcastic over-praise – as in “Really? Obama was the only one to suggest the Navy Seals?” – and as such seems the usual deployment of one of Rush’s favorite attack lines: the media’s over-praising of and over-focus on Obama.

But I have to say, after having watched the video of the same opening monologue, and hearing the lack of irony in Rush’s voice when he says "Thank God for President Obama," that the monologue sea-saws in a way that seems to indicate a rather poignant confusion within Rush himself.

Another goes with sincere:

As a former conservative and regular listener to Rush's show, I sat watching the clip and waiting for the punch line.  There was none.  Even Rush Limbaugh realizes the huge significance the killing of Bin-Laden has both historically and to the psyche of  the nation.  He may be a lot of things but he's not that stupid.

Another "listened to the entire passage":

What Limbaugh is attempting to do is posit the intelligence and the military apparatus as remnants of the Bush administration, which Obama wisely kept in place.  The sarcasm comes into play when describing Obama as “the only one in the world” who could have thought of using Seal Team 6 and Special Ops.  So, if you piece it all together, it was the Bushies who did the real work through torture at Gitmo (you are correct there is absolutely no support to this claim) and any imbecile could have stood by while the CIA and the Seals went about their work.  It's nuanced derision, and not the genuine praise you seem to think it is.

Another is still listening to Rush's show:

He is mocking the whole thing in the 3rd hour. He is an asshole.