It qualifies as peer-to-peer in my book as long as you've got enough people to run server nodes so it couldn't be shut down.
It's not so simple some external mechanism needs to prevent sibyl attacks, otherwise I spin up tons of 'servers' and do nasty things.What I'd call it depends on how that determination happens in practice. I don't believe that something I'd call "peer-to-peer" e.g. listen to any server you find would actually work and be secure. One option is Centeralized: e.g. OpenCoin ends up with defacto or dejure control over the lists this would make the trust scarce and worth behaving to keep, but would also allow shutdown and takeover. But it's not the only option, and I don't know what name I'd give the other ones.
E.g. imagine Bitcoin largely as it exists today, but no POW (difficulty=0)... and no description in the protocol over which chain to accept just "get it from a trusted party". Is that peer to peer? Centerlized? What is it? It's not a question you could answer without knowing how people would choose their chain in practice. Ripple specifies more than that, but I think not enough more for me to say what kind of system I think it is.
I wouldn't even call ripple peer-to-peer between the servers, simply because not all "candidate peers" are equal some external process makes some peers important and some irrelevant. That might meet the English definition of peer to peer but not the technical one. Call it "friend to friend" might avoid the overloaded meaning, but it would be odd to call relationships which are primarily between large banks "friendships".
It's peer to peer, but only inside an exclusive club. The nature of the clubs' exclusions define the system more than anything else. For example Paypal's infrastructure could be called "peer-to-peer", but to be a peer you must be part of paypal. Hm. Maybe I should call Ripple's general class of consensus algorithm "Crony Consensus".
(FWIW, Tor is distributed but not decenteralized. This is regarded as worry-some by many, but at the same time, the influence of the tor directories is inherently transitory. Tor doesn't represent stored value. But Bitcoin and Ripple do, the tradeoff is different)