THE NY Times has an op-ed by renowned psychologist Steven Pinker espousing his theory of why Roberts screwed up the oath:
On Tuesday, Chief Justice John Roberts joined the Flubber Hall of Fame when he administered the presidential oath of office apparently without notes. Instead of having Barack Obama “solemnly swear that I will faithfully execute the office of president of the United States,” Chief Justice Roberts had him “solemnly swear that I will execute the office of president to the United States faithfully.”
How could a famous stickler for grammar have bungled that 35-word passage, among the best-known words in the Constitution? Conspiracy theorists and connoisseurs of Freudian slips have surmised that it was unconscious retaliation for Senator Obama’s vote against the chief justice’s confirmation in 2005. But a simpler explanation is that the wayward adverb in the passage is blowback from Chief Justice Roberts’s habit of grammatical niggling.
Among these fetishes is the prohibition against “split verbs,” in which an adverb comes between an infinitive marker like “to,” or an auxiliary like “will,” and the main verb of the sentence. According to this superstition, Captain Kirk made a grammatical error when he declared that the five-year mission of the starship Enterprise was “to boldly go where no man has gone before”; it should have been “to go boldly.” Likewise, Dolly Parton should not have declared that “I will always love you” but “I always will love you” or “I will love you always.”
As for me, I have rarely separated the verbs in my writing and find that to never split an infinitive is usually quite easy.