ZEN:
"We have not spoken to them and we would not speak to them," a Zen Internet spokeswoman said. The firm specialises in business broadband. "Zen Internet will continue to monitor OIX [Open Internet Exchange, Phorm's advertising network] with regards to protecting its customers."
http://www.theregister.co.uk/2008/02/29/phorm_broadband_isp_targets/
Plusnet:
A spokesman at the Sheffield-based provider could not be reached today, but product manager Ian Wild told subscribers in its forums: "We'd certainly do an opt-in or opt-out with something like this if we ever did it.
"Just to say again though, I'm pretty neutral from a product manager perspective as long as our customers are happy, and from a personal perspective it's something I might well choose to opt-out of unless the reason not to was very compelling."
http://www.theregister.co.uk/2008/02/29/phorm_broadband_isp_targets/
From the link you posted, "We have not trialled Phorm and have no plans to do so." is to me a fairly definitive, we won't be trying it.
Unless of course they were going to get BT to trial it, and then Plusnet would implement it without doing a trial. That's really the only other way our friend Neil could get out of that one.
Anyway, the balls in PN's court, are they going to confirm they will not implement it, or confirm there is a real possibility that they will use Phorm.
I wouldn't expect an open and honest answer though, as too many people may possibly leave.