William Safire on why President Bush—among others—says "nucular" instead of "nuclear":

We are dealing here with a phenomenon called metathesis (pronounced mih-TATH-uh-sis), the switching of two adjacent sounds within a word. Many of us replace an unfamiliar sequence of phonemes (the smallest units of speech sounds) with a familiar one. The only other common English word that rhymes with nuclear is the unfamiliar cochlear. But in our spectacular language, there are dozens of words like secular, vascular, jocular and molecular, and our brains are tempted to make nuclear fit that familiar pattern. (I can even hear a variant ''cochular option.'')

There are at least two occurrences of "nucular" in The Simpsons.  One is when Marge mispronounces the word "foliage" and Lisa corrects her:

Marge: Next to Spring and Winter, Fall is my absolute favorite season.  Just look at all this beautiful foilage.
Lisa: It's not "foilage," mom, it's "foliage."  Foo-liage.
Marge: That's what I said, foilage.  It doesn't take a nucular scientist to pronounce foilage.
Lisa: [growls]

The other time "nucular" makes an appearance is in the episode where Homer is assigned to weekend warrior submarine wargames, and we hear Captain Tenille (!) speaking to Homer:

Capt. Tenille: Simpson, as you have experience in a nuclear power plant, you can serve on a submarine.
Homer: It's pronounced 'nucular'.  Nucular.

The the guide for that episode mentions that "nucular" is also mentioned in the episode where Bart gets an elephant.