I've been able to dismiss other Followers, and then get them back later because they are better suited for certain missions. So I found it useful to dismiss Preston (who adds some useful firepower for combat situations) and send him to Sunshine Trading Co-op. But later when I go there to get him back, all I can get out of him is "Are we ready to take the Castle?" No "Follow me" option. Does this mean that I've lost him as a Follower for good?

I have a similar problem, although it took place after the Castle mission. I can't open the dialogue tree; every time I try to interact with Preston he just tells me that he trusts me and goes back to tending the crops. Maybe he's trying to live the quiet life now.

I've done everything to Preston except reverting to an old save, even being so considerate as to massage his face with the butt of my shotgun, but Preston is unresponsive. So yeah, you've probably lost him as a companion until it's patched. He still seems able to give quests and turn them in, though.

I encountered a similar bug with Hancock, in being unable to open the dialogue menu. He complains about a part of the Big Dig quest that I already completed instead of speaking to me about current events, which also makes it impossible for me to progress with the Silver Shroud quest.