But if they did have a trademark case, this would likely strengthen it. If you benefited from a referral program, huddle.net could prove you profited from domain infringement.
If they do, however, I think they might chill with the fact that I would be giving them some customers. Even if not, it's a good thing that I'm not living in the US at the moment (and not moving back until late 2010).
It is a nice service though, and awesomely simple.
IANAL, however, just a programmer.
I don't think huddle in the UK would be considered to lack the distinctive quality that makes it suitable to be a RTM but perhaps in the USA?
TM are registered in an area of business, as companies grow they tend to fill [almost] all areas, I'm sure you can get MS clothing, MS food, MS surfboards, ... if huddle isn't registered and can be then registering it is fine but as mentioned it does not prevent current use from being continued IIRC.
* I'm clearly not a TM lawyer and this ain't advice.