Hey I promised I'd research The Mark of Athena and tell you what I find out. So...
Wisdom's daughter walks alone,
The mark of Athena Burns through Rome.
Is what Ella states in the Son of Neptune. Annabeth is PROBABLY the one in the Mark of Athena. But we don't know for sure. This would be the PERFECT opportunity for Rick to trick us. We'll expect Annabeth and get some one else. So don't take a bet off another character. Mark of Athena. I 'm going to take a moment to think. The Mark of Athena is..
Well there's plenty of other's but this seems like the most likely possibility. A burning olive tree? Hmm...
Now for what I found out...
The Mark of Athena is an upcoming book written by Rick Riordan that is scheduled to be released in Fall 2012, according to the final page of The Son of Neptune. It is the third book in the series, The Heroes of Olympus. It will continue the adventures of the seven demigods of the second Great Prophecy and their quest to stop the giants - the sons of Gaea - in Rome. The Greek and Roman heroes will try to close the Doors of Death and save Nico di Angelo, who has probably been taken captive by Gaea's forces.
Annabeth Chase is to have an important role in this book, as Ella said 'Wisdom's daughter walks alone, the mark of Athena burns through Rome." Juno also told Percy 'the one who will cause the most trouble is the one closest to you-- the one who hates me most' though Percy angrily ignores Juno's advice.
This is from wikipedia. That was one of the only sources I could find on it. So I guess to learn more we gotta wait a little while.