Where did you hear this, officially? Because it contradicts what the US Consulate in Sydney told me last year. Their clarification was that each entry into the USA on the VWP was for a maximum of 90 days, and this 90 day period included time spent in Mexico/Canada, and if you end your VWP stay in the USA upon entering Canada, there's nothing stopping you from re-entering the USA under a new VWP entry at a later date. This advice also agreed with the informal advice I received from a friend of a friend who worked for DHS in the USA. I found the level of knowledge amongst US border officials very variable - once had to spend 15 minutes explaining the concept of a working holiday visa to a bored and suspicious guard in Detroit. It seems like their biggest worry is people entering the country to work illegally, which is a hilarious concern in Detroit with its 25% unemployment. It'd be awesome if the US Embassy could address this question on their website, because it causes a lot of worries for Australians heading to Canada who just want to do the right thing.