My layman's prediction is we will be able to measure IQ in the future by some means of scanning the brain's process of building connections and nerves. An aptitude test is simply measuring the side effects of IQ.
"Mental tests may be designed to measure different aspects of cognition. Specific domains assessed by tests include mathematical skill, verbal fluency, spatial visualization, and memory, among others. However, individuals who excel at one type of test tend to excel at other kinds of tests, too, while those who do poorly on one test tend to do so on all tests, regardless of the tests' contents."
"[G factor] is a variable that summarizes positive correlations among different cognitive tasks, reflecting the fact that an individual's performance at one type of cognitive task tends to be comparable to his or her performance at other kinds of cognitive tasks."
There's some interesting imaging research over the past decade or two suggesting that if you want to reify IQ as something physical, it might wind up being something along the lines of global connectivity - how well and efficiently distant brain regions can communicate and coordinate activities.
What do you think?
pd: strange that this is the first time creativity is mentioned in this thread.