I actually sowed mine in Spring, a few years ago. I can't remember whether they flowered the first year.
The T&M seed catalogue does advertise verbena as first-year flowering perennials. That's reasonable because they do flower late in the year (now).
The T&M seed catalogue (2012 season) has an unusual footnote, just beneath verbena, saying that verbena seed is only despatched between March and May.
In nature, and this is probably true of self-sown seedlings, the seeds fall to the ground when the flowers have bloomed and the seed has ripened. I'm not sure exactly when that happens. Maybe in a few weeks time, or perhaps the seed stays on the plant through the Winter. They are flowering now, so this year's seed is obviously not ripe yet.