A weighty trade mark issue for the US courts

The IPKat has learned from Reuters that Weight Watchers is suing Nestle for trade mark infringement in the District Court of Southern New York. Weight Watchers claims that Nestle has displayed trade marks relating to Weight Watchers and its points system on its Skinny Cow ice creams and Lean Cuisine ready meals. Weight Watchers fears that this may confuse consumers into thinking that it has endorsed Nestle's products. Nestle meanwhile runs its own weightloss programme, Jenny Craig Inc.

This looks to the IPKat like a rather unconventional interoperability case. Can you use someone else's trade mark to tell them that you food fits into their diet programme? Even if you can, should you do it in a way which doesn't cause confusion?