I travel across the border 15 times a year for business and 3-4 times for pleasure. My laptops are supplied by my US-based employer. They buy them and I bring them back with me in my briefcase. In years of doing this, never once has anyone said anything. My suggestion is to take an old briefcase or knapsack so it looks like it's been used and put it in there.
As for warranties, mine's a Dell, so others could be different, but Dell Canada honours the warranty up here no probs.
Go for it, don't sweat it.
Cogito ergo BMW...I think therefore I drive
Currently driving Jag but may have to get back to BMW!