When we checked in at the US Customs building north of Ely, the Customs official there said that the US Customs agency does check with the Canadians to determine who was in the Park...and then they cross-check to see if these Americans checked back in with US customs.  He said you can receive a hefty fine for not checking in with US Customs.  I'm not sure exactly how the "checking" with the Canadians gets done...but it's so easy to do (stopping in at the US Customs office), why take the chance?  It takes maybe 10 minutes.  

prouboy