Tablets in general and the iPad in particular are a fairly recent phenomenon as widespread devices for consuming or interacting with information. Studies that examine the long-term effect on kids cognitive abilities when using a smartphone or tablet apps take 3-5 years to complete, and the iPad has hardly been around that long.
There is some research done on educational apps with an iPod touch, which show 5-year olds picking up vocabulary 27% faster with an app, whereas 3-year olds, whose brain synapses have just reached maximum level, picked up 17% faster using another one.
Researchers are still on the fence whether a long-term interaction with an iPad can lead to attention deficits in kids in this synapse-growing age up to 3 years, like long TV-watching was blamed to cause. The tablet experience is much more interactive, they say - kids look away from the TV more than a hundred times an hour, whereas with an iPad app their attention is so engaged that they zone out completely until they finish the task at hand.
Studies show that 29% of 2-4-year old kids, and 52% of the 5-8 crowd have used a touchscreen device, be it a smartphone or a tablet like the iPad, and parents' unscientific observations show that they picked up vocabulary very quickly due to the interactivity of the games and puzzle apps.
As for the long-term effects to attention and brain development, these are still to be examined, but in the meantime we are likely to see more and more parents using the iPad or another touch screen gadget as a pacifier.