Are there laws about how close military aircraft can fly to passenger jets, and are they enforced? Yesterday I was on a passenger plane flying at 35000 feet from Denver to JFK on a fairly clear day. Just west of Sioux City, we made a relatively sharp, but short, right turn for whatever reason (may be unrelated). I happened to be looking out my window and caught what appeared to be a military fighter jet (presumably from the Air Force base there) flying in the opposite direction. It blew past my field of vision in a fraction of a second, slightly below us and off to the right, close and fast enough to startle me. It left a beautiful set of trails but, while my distance judgments aren't very good from a plane, appeared to be no more than ~1000 feet away from us (based on its size, I imagined it to be about 4-5 NYC city blocks away, that's my only reference). The passing aircraft's trails were dead straight at least as far back as they extended before dissipating. I didn't get a chance to ask the pilot about it after the flight.

My only knowledge of separation laws is from wikipedia, which seems to state a 2000 foot limit above 29000 feet.  I absolutely might be misjudging distance, but this aircraft seemed to be well under a half a mile away. So my question is: Are there a different set of separation laws for military aircraft, or perhaps are existing laws just not as strictly followed / enforced by the military? Would the pilot of one of the aircraft expect some sort of reprimand later? Was this a normal occurrence or was it a close call?
This is somewhat speculation, but here's what I think happened:

You were flying at around 35,000 feet, in RSVSM airspace. The fighter jet was correctly on track 1000 feet below you. Without any other guides of distance, you tried to judge its distance by apparent size compared to real size. You might have underestimated the size of a fighter jet; they're typically about 50 feet long. I know I, for one, always thought they were much smaller. Thus what looked like "slightly below" was actually 1000 feet below you.

As for the more general question you posted: the military can get as close to other aircraft as they damn well please, through a procedure called Military Assumes Responsibility for Seperation of Aircraft, or MARSA (thanks to fooot for pointing this out!). Generally this only happens with a civilian aircraft during an interception. I don't know the specific procedures that are followed, but I know that in the past intercepting pilots have gotten close enough to pick out individuals in the cockpit.

As for enforcement, the question is kind of null. Generally, if civilian pilots violate separation without putting anyone in danger, the air traffic controller will just yell at them for a bit, but it does depend on the case. The FAA doesn't have much authority over the military, however. If the air traffic controller passed the information on to the military, perhaps there could be some sort of corrective action, but at this point I'm just guessing.