A history of the Sunderland Football Club at
http://www.answers.com/topic/sunderland-a-f-c uses both of the phrases "pipped to the post" and "pipped at the post".
The relevant sentences are
Sunderland's first season in their new home yet again involved them being pipped to the post, as they
finished third in a tougher-than-usual Division One.
In the 1999-2000 season, Sunderland finished seventh in the Premiership ? their highest finish since 4th place in 1955. Again the team was pipped at the post on the last day of the season, this time missing out on a place in European competition.
The Oxford English Dictionary has under "pip v(3)" definition 1c
c. To anticipate or forestall (someone) in a
particular activity, circumstance, etc.; spec. in phr. to pip at (or on) the post, to defeat by a narrow
margin at the last moment; also ellipt.
This seems to say that "pipped at the post" and "pipped on the post" have the same meaning, but how about "pipped to the post"?
It seems a little strange that a writer would use the two phrases within a paragraph or so in the same discussion without having some different shade of meaning in mind.
Is there a difference, however slight, in connotation?