The Internet Advertising Bureau's latest numbers for the state of the US interactive market are perfectly respectable. Spending in the first quarter, at $4.9bn, was up $1.1bn on a year earlier. But extrapolation junkies take note: the growth is now arithmetic, not geometric. That hockey stick you put in the latest over-enthusiastic financial projection: it should be straight. After the jump, the more sobering chart, showing a slight deceleration in the growth.