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That is exactly what the 'smartfee' code in the reference implementation does.
RE: where does the market information come from:
Like any market, it comes from the collective action of lots of individual decisions. Different wallet software has different fee policies, and there is already a little bit of "I sent a transaction using wallet XYZ and it took FOREVER to confirm, WTF?!?" (or "why does exchange/wallet/service ABC charge me such high transaction fees").
As wallets mature, I expect them to specialize ("Save on Fees! Use UberWallet!") and/or compete for best cost/speed/reliability/predictability.
The default for the reference implementation will be "follow the herd" -- but the price will be set by the minority of people 'at the margins' who REALLY want their transactions to confirm quickly or REALLY want spend as little as possible on transaction fees. They will set -paytxfee and/or -txconfirmtarget to override the default behavior.
And "they" are likely to be high-volume-transaction-creators-- like exchanges (probably want their transactions to confirm quickly; fewer customer support calls) or watch-a-video-get-a-few-bits services (probably want to cut costs any way they can, don't care if their customers have to wait a while for a withdrawal to get confirmed...).
RE: sybil/isolation attack:
Again, not a likely attack. You would have to:
1) Find some high-transaction-volume service and identify all of their bitcoin-network-connected nodes
2) Control ALL of those nodes' connections (expensive to do reliably with the 'connect out to 8 random peers' rule) FROM THE BEGINNING OF TIME (well, beginning of when it started running the smartfee code).
3) Let that node see only extremely-high-fee transactions (there aren't very many of those, so you'll need to manage to control the node's connections for a while).
4) Expect the node's operator to send a lot of transactions and not notice that they were paying abnormally high transaction fees.
If you are running a high-transaction-volume service you probably already have several connections into the bitcoin p2p network because you have probably already been the target of a distributed denial of service attack....
Definitely not an issue for Bitcoin-Qt, because you're shown the total amount + fees you'll pay before every transaction.