I must agree that getting out the word on the attempted takeover is the best tatic. Seemingly everyone loves a David vs. Goliath story, so you should have no problems finding someone to pick up your story. I guess the watershed comes when you have made more bad PR for the large company than having an unoffical website which someone might erronously go to.
Just out of curiousity, how many people do you think really give up looking for pepsi.com (to steal someone's earlier example) after going to pepsi.org by accident? Enough to actually cost Pepsi any sales? The only problem I can see Pepsi having with "pepsi.org" would be if the .org existed solely to criticise Pepsi. I'm not saying Pepsi would be justified to then seek the pepsi.org domain name in that case, merely that it would be in their best interest to do so.
Situations that fall into the "Well, it looks sort of like our name, so we figure we will get some lawyers to get it for us" category always make the company look greedy if/when the story breaks of their attempted takeover, which would suggest that perhaps it isn't the best of ideas attempting domain name takeovers in such cases.
I imagine that the argument raised by companies most often would be the need to protect their trademarks or lose them, but doesn't that only apply to products of the same type? I don't see how something such as a web site with the domain of pepsi.org could be brought up in a court as proof that Pepsi has become like Kleenex et al.
Personally, I'm more worried about how to protect a domain name from a company which comes much later than the registration of the domain name (a la etoy.com/etoys.com). What, short of trademarking a domain name yourself, can you do to protect your domain name from future corporations coming along with an "official trademark" filed 5 years after your domain, and demanding that you give them your domain? Can you hold up a 'whois'
output in court to prove that you have had the name for 5 years (or whatever)? And more importantly, how can you convince a corporation that it is a guaranteed victory for you, therefore never involving any pricey lawyers?
On a somewhat related note, does anyone else get a selection between YAHOO.COM.IS.TRYING.TO.STEAL.YAHOO.VU.HOW.ACIDULOUS.COM and YAHOO.COM when they do a 'whois yahoo.com'? It certainly struck me as an interesting though limited way to spread a message about a case such as this.