Someone asking for help on the bitcointalk forum for a new venture? Happens almost daily. Someone asking a question on SO about how to access Tor? Ditto.

You don't discover who "Dread Pirate Roberts" is from this. But you do discover these types of things pretty easily AFTER the NSA tells you who DPR is.

Obviously there's lots of ways that guessing DPR's identity might allow someone with the FBI's resources to unmask the Silk Road server, though I don't know enough to know whether the forum post on its own would be considered sufficient evidence for a warrant to bug all of Ross Ulbricht's online activities. A lot of the more damning evidence for Ross Ulbricht as DPR (IP logs, the connection to the counterfeit documents, hostname of his personal machine, etc) seems to come from forensics on the captured server image. Analysis of Tor traffic doesn't seem like an implausible hypothesis, especially because that's a capability we'd be expecting the FBI/NSA to be developing anyway.