Brit touring the US and Canada

I am a British guy looking to tour North America in 2014, including the US and Canada on the same trip.

Plan is to spend 3 months in Canada then cross border into the US, and spend 3 months there, then fly home to UK.

It would be a "see where the road takes me" kind of deal so I wouldn't have a return flight booked at time of travel.

Firstly, am I permissible into the US from Canada?

And would either Immigration service (Canada or US) be concerned that I didn't have a return flight booked?

Any answers and other relevant advice would be appreciated.