The letter states the following:
I regret that I will not attend any mayoral forums until the official filing deadline for mayor has closed and the full field of aspirants is known.
Holding mayoral debates before all candidates are declared is undemocratic and misleads voters.
In particular, Mayor Dellums has yet to announce his intentions as is his right as an incumbent. The mayor is running for re-election until he says he's not, or when filing closes in August. Ron Dellums has served our community for almost four decades. He's well entitled to such consideration.
Presumably, Perata's statement also applies to Oakland At-Large Councilmember Rebecca Kaplan, who's not offically said she's running for Mayor, even though she's reportedly participating in the Thursday forum.
The letter is also here.