Within months of its birth the infant iPad surpassed a 27-year-old product, the Mac, in unit shipments. In Apple’s fourth quarter, people bought 4.2 million iPads to 3.9 million Macs. The shift is forcing Wall Street to do things like lump iPads in when calculating Apple’s total PC market share. If sales track expectations next year, Apple will have its usual dinky 5% PC share without iPads, but a major-league 12% share with iPads."
Squeeze the Apple Juice, our take : no cannibalization but rather the 'halo effect'. The halo effect is when great Apple products like the iPad, iPhone cause people to look at Macs and often buy them. Ipad and iPhone and before that the iPod introduced Apple to a whole new audience. Apple said that 50% of Mac customers are new to Mac.
As for growth rate slowing: Apple quarters are not equal , some quarters like Christmas are higher and quarters after a big mac refresh usually show better sales. The trend is positive for Apple. Various analysts looking at IDC data etc have discerned that Mac sales are outpacing PC sales.