When a pilot ejects, what happens to the ejection seat? Does the pilot stay in the seat until he lands, or does it get disconnected from the pilot somehow?
It depends on the seat. Most modern seats, including the ACES II system common in US Fighter Jets, fall away from the pilot at a pre-determined altitude. [src]
Once out of the plane, a drogue gun in the seat fires a metal slug that pulls a small parachute, called a drogue parachute, out of the top of the chair. This slows the person's rate of descent and stabilizes the seat's altitude and trajectory. After a specified amount of time, an altitude sensor causes the drogue parachute to pull the main parachute from the pilot's chute pack. At this point, a seat-man-separator motor fires and the seat falls away from the crewmember. The person then falls back to Earth as with any parachute landing.
Generally, according to that link, this occurs at just over 100-200 feet above ejection height. However, if you eject at high altitude, such as Captain Scott O'Grady did (26,000 ft), the seat will fall away on a sensor (either 10,000 or 18,000 feet, I don't recall which). The chute may open automatically, or if it is not set to at that height, you can then pull the chute manually, or wait until its set height to open.
I ejected from an FJ-4 at 17,000 ft at 410 kts IAS. I was very briefly unconscious. When I woke up I was head down and looking up between my legs at an object drifting away from me which I saw as my parachute back pack. I could see the tabs down each side side of the pack as well as the pin attached to the D ring. I fell about 13,000 feet before I looked at the D ring. I occurred to me that I might as well pull it even though I still thought I had somehow lost my chute. I felt a minor shock and looked up to see my parachute open. Then it dawned on me that the object I thought was my back pack was the ejection seat. As I remember separating from the seat was simply a matter of the pilots body and the ejection seat having differing response to the wind experienced after ejecting.This incident took place in July of 1957. My back will probably quit hurting before many more years.
My only reference is a T38 flight manual I happen to have. That seat has a "man-seat separation system" that is a separate pyrotechnic device that unlatches the straps holding the pilot to the chair after clearing the aircraft. (The pilot wears the parachute, which nestles into a cavity in the back of the seat.) The manual exhorts the pilot to try to manually disengage from the seat, but says that the man-seat separation system can rarely be beaten. There's nothing in the manual that says the seat has a separate chute of its own.