On that Thursday night, according to John, Jesus told his disciples the greatest agape was to die for one's philoi, and that if they showed agape for one another they would be his philoi and remain in his agape. Then, a couple of weeks later, the resurrected Jesus asked Peter, "Agapao me?" Peter replied, "Lord, you know philw you."
Therefore - according to Jesus, according to John - the philos is the bearer of the agape. Therefore - according to Peter, according to John - 'phileo' is a good way to respond if you're asked to verify your 'agape'. The words are complimentary, but not interchangeable. There's a reason why each one was used.
The philos is the foremost bearer of agape. That means Peter's (initial) response was an emphatic YES. There's certainly more than just that going on between John's 15th and 21st chapters, between Jesus and Peter, but I think that's the main part of the meaning behind these two "loves".
For the rest of my thoughts on John 21, scroll through my June archives for 2009. Or check out this summary. And remember, we are his philoi. Let's abide in his agape.