I think Jonesboro is well on its way to reaching the 100k mark. Infrastructure is an issue, but not as much as one might think. The city is well-known to those who go/went to college there, and has a good business climate. As mentioned earlier in the thread, Jonesboro is also undergoing some important internal improvements to make it a more livable city for its residents... this will continue to foster a strong growth pool based on regional leadership. From a sociological perspective, Memphis and Jonesboro appeal to different types of people for different reasons, which means Jonesboro will continue to grow from its current population set (those who prefer a mid-sized city with decent jobs, and might be intimidated by a larger city like Memphis).
In reference to the next city to reach 100k, most definitely Ft. Smith. It's not growing at the same rate as Fayetteville or Conway, but Ft. Smith is gaining more population within its city limits. Conway has Little Rock, Maumelle, and everything else to compete with, and Fayetteville has to compete with Rogers and Springdale for new residents. Ft. Smith only has Van Buren, so more people are settling in the larger city. Also, NWArkansas is beginning to level off from the ridiculous boom years, and Ft. Smith remains steady in growth.