[lutorm]

Being Swedish, I've always had to surrender my I-94 upon leaving, though that's always been by air (including at SFO). (And there's always a warning on it that says essentially that "if you fail to surrender the I-94 we will think that you've remained illegally".)

[abhijitr]

AFAIK, they don't take your I-94 if you're traveling to Canada or Mexico for under 30 days.