On my mexico trip, I forgot about insurance till I was at my Uncle's house in McAllen, TX. At 9:00 at night I searched around on the internet, but couldent complete the form b/c they didnt list my bike (xr650l) so I called them at almost 10:00 and at 11:00 he emailed me the insurance form, I printed it out and was good to go. Cost around $10 per day for my trip.
But I was never ever ever asked for the insurance info. Not by customs, not by the cop at the checkpoint after the 30km (or whatever it is) free zone, not by the mililtary when they searched me. So i am assuming no one will ask you for it unless you get in a wreck. But better safe than sorry I guess.