It is technically possible to build a coaster taller than 1,000 feet tall today. Wherever you live, there are probably broadcast sticks that high somewhere nearby. It isn't a technological problem. The problem is that as the height goes up, the space requirements go up. Notice that Millennium Force only has one hill between the station and the turnaround? The extremely steep lift hill helps with the footprint a bit, but the basic problem is simply the sheer amount of ground needed to make a big coaster work.
Not to say that it can't be done. But look at it this way: as the height goes up, so does the cost, while the speed only increases with the square root of the height...and we are going to bump into some limitations on how *fast* the coasters can go before wind becomes a really big problem. In fact, as the coaster gets bigger, all of the problems get bigger.
So consider the park's perspective: If the park builds a coaster that is 310 feet tall, it will be the World's Tallest. The next coaster might be 325 feet, but it will also be the World's Tallest. That leaves open the possibility of building one 335 feet or 340 feet or 350 feet. The park gets just as much benefit from breaking a record as it does for smashing it into itty bitty pieces. But the added benefit of going *way* beyond the record is so small that it's not worth bothering with given the hassles.
--Dave Althoff, Jr.