A complicating point here is that the Kerry/DNC lawyers have written to broadcasters warning them not to run the ad, not to the SBV for producing it; apparently, if they sue, they will sue broadcasters who run the ad. A few posters on other threads have pointed out that the liberal media will now have a plausible excuse to refuse the ad. Luckily, there are at least some conservative outlets that will run it, so these will be outlets that will be sued.
"In a civil suit, both sides have the right to discovery:"
Then the SBV should sue Kerry and the DNC for what is said of them in that fax.
Thank you both for your input. The Kerry/DNC warnings to the media outlets are probably bluster to scare them, plus as maryz points out it gives them an excuse not to run the ads. Hopefully there are enough independent stations left, run by small businessmen who are either conservative or principled enough to call their bluff and run the commercial.