According to http://www.sfgate.com/cgi-bin/blogs/sfgate/detail?blogid=14&entry_id=34812, the Chief Justice did in fact make a mistake while reading out what Obama was supposed to repeat. He misplaced the word "faithfully", which I think is what could've confused Obama slightly.
Here's the transcript, taken from the same site (Obama started talking a bit too early the first line):
[quote]ROBERTS: I, Barack Hussein Obama...
OBAMA: I, Barack...
ROBERTS: ... do solemnly swear...
OBAMA: I, Barack Hussein Obama, do solemnly swear...
ROBERTS: ... that I will execute the office of president to the United States faithfully...
OBAMA: ... that I will execute...
ROBERTS: ... faithfully the office of president of the United States...
OBAMA: ... the office of president of the United States faithfully...
ROBERTS: ... and will to the best of my ability...
OBAMA: ... and will to the best of my ability...
ROBERTS: ... preserve, protect and defend the Constitution of the United States.
OBAMA: ... preserve, protect and defend the Constitution of the United States.
ROBERTS: So help you God?
OBAMA: So help me God.
ROBERTS: Congratulations, Mr. President.[/quote]
Just thought I'd share