First, more suits will be sold because many mishies from Utah have only been wearing white shirts and ties and now need them for their missions. Of course, smart ones will buy on E-Bay.
And in a couple of years, after the initial surge, the number turning 18 will be the same as the previous number turning 19 and the number will drop back close to where it has been, with the only exception being a few more sister missionaries since fewer will have found a husband yet and will go on missions.
So the long term end result is the status quo for elders and a few more sister missionaries. The huge increase and visibility is mostly a temporary anomaly they've created.
Actually, I think the number of young men will increase long term. This is because they were losing so many between 18 and 19. Too many going off to college and learning to think for themselves. If the corp can get them on missions, then they are more likely to be locked in for life. They will come home, go to school (hopefully), get married in the temple and start popping out new revenue streams. With the age lower for girls, they will also make up a much larger percentage of the total missionary force.
On the other hand, I predict that the number of convert baptisms per missionary will drop a bit over the coming few years. It has dropped nearly 50% over the past, from a high of approx. 8 per year per missionary to 4.3 per year per missionary in 2003, recovering a bit to 5.1 last year (2011). My bet would be that the number drops back to around 4.3 - 4.5 in the coming two years. No matter how many missionaries polish a turd, it remains a turd. It costs the corp materially nothing to staff a missionary force, so it is all upside.
Actually, the reason for the great increase in female applicants is that the number turning of age and available is triple the normal number. The number of males is an increase in only 75% because half of the eighteen-year olds will not be eligible until they graduate from high school, meaning with a time-averaged delay of three months. As pointed out, these are temporary increases and not only are a problem for colleges this spring and fall but also a mission housing problem. There will also be an unusually large influx of students after the bubble comes off missions. This is just short of what happened to colleges after the end of WWII. I wonder if BYU is ready for these changes.