Vouching for (or against) the reputation of a bitcoin user Since it's possible to link non obfuscated transactions to a "user" does is make sense, or it is possible for anonymized users with an established transaction history to "vouch" for another?

I'm thinking this would be similar to EBay's seller rating system, where people can prove they did a transaction with someone and then comment on the experience accordingly.
There is already three rating systems in existence from different trading platforms: #bitcoin-otc , Bitmit and Silk Road. Bitcoin users actively using their rated OTC identity to make their words more trustworthy.

I believe integration of rating system into Bitcoin Network won't be beneficial and developers will not ever try to implement that. It is possible that in future obfuscation will be enabled by default, at least it is feasible to do it : Automatic Coin Mixing Idea and P2P coin mixing.