My kids are 4 & 2 and we don't have a TV, yet I still see the pernicious effects of advertising in them. We have several DVDs, and they recognize the logos of those characters (mostly Bob the Builder & Thomas the Tank Engine), and every time they see anything associated with them (like the HIT Entertainment Logo, which is the producer of those videos, as well as a lot of other kid videos; or Nick Jr, which does Dora and Little Bear), they holler out what it is. They watch TV at my mom's (she has a DVR and records Dora, Little Bear, Tom & Jerry, etc.), so they don't get just a whole lot of advertising, although T&J does have regular commercials. So far, they haven't seemed to have been affected by it, but a lot of the commercials they've seen are not exactly for their age group. Still, whenever we go to Walmart or see stuff online, my kids know Elmo, Dora & Diego, Thomas (and all the trains), Little Bear, etc., and my older son has started asking for all the stuff that we see when we go to the store.
My sister's kids have the "I wannas" a lot more, but they are also several years older, have outside friends who are also into stuff, and watch **a lot** of TV. When they were my kids' ages, though, they were a lot more susceptible to ads. My sister & her husband also gave in to everything, spent tons of money on Christmas & birthdays, and basically have let their kids believe they can get anything they want. But I know a lot of their desires were also fueled by what they saw on TV.
TV advertisers know what sells. They know how to get your kids to want what they see. I still remember seeing Barbie commercials when I was a kid and wanting that particular Barbie (vacation, Day-to-Night, doctor Barbie, whatever).
I remember seeing something years ago about research done on preschool-age kids, in which they showed them a couple of dozen different logos -- just a single letter out of them, like the "M" from McDonald's, the "W" from Wal-mart, or the "A" from the laundry detergent "All" -- and it was incredible how many ones they knew. They knew some that I didn't know!