The criminal complaint[1] mentioned the FBI searching for the earliest mention of Silk Road. The earliest they found was a user called 'altoid' advertising it in two separate places (including bitcointalk) around the same time. Later, 'altoid' posted a second time on bitcointalk searching for an "IT Pro" and publicly listed the email address "rossulbricht@gmail.com" as the contact info for the gig. This was also the email address he used to register his Stack Overflow account.

[1]: http://www1.icsi.berkeley.edu/~nweaver/UlbrichtCriminalCompl...