Oh, I know, it's soooo depressing! I had the great pleasure of watching my toddler stealing bits of asparagus off my mother-in-law's plate. It was wonderful. She thought I was doing such a great job with him. He also ADORED steamed brocolli. Brown rice, hummus, all kinds of healthy things.
And as long as I kept making it ALL THE TIME (like 2-3 times a week), he'd eat it. Then, maybe I'd go for a week or 2 without serving it and WHAMMO! - suddenly it's a foreign, unidentified food.
It helped me to understand that not only is this normal, is biologically imperative. Throughout human history, children at this age were starting to explore the world, and sometimes wandering farther from their mothers. The children who put any old thing in their mouth (you may recognize this because she was probably doing it last year) were more likely to come across something poisonous or dangerous. The children that didn't eat anything unless it was VERY familiar lived. (I swear i'm not making this up - I have a biologist friend and we have these great discussions. Also read it in "Our Babies, Ourselves")
So, this tendency towards being a picky eater has tended to reinforce itself thoughout our history. They are hard-wired to do this for a while - it has nothing to do with what you are doing right or wrong. Instead of fighting it, just think "That's my girl - developing exactly the way she should be - she won't be eating any poisonous berries! Or, you know, anything else..."
My own sister went through a Cheerios-and-milk-only phase for WEEKS! And I once read a great story about a mom whose son would ONLY eat white food. (He was older, too, like 7-8). Mashed potatoes, white rice, milk. And what really drove her crazy was that she was REALLY into food - top editor at Gourmet magazine or something like that. It just KILLED her - she wanted to set him up with a machine that pumped vegetables into his stomach while he slept. Then one day, they were at a sushi restaurant and she asked the waiter to put a little... I think it was some sort of squid sauce - on the rice. Then she had (for a while) a kid that would eat white food and squid sauce. Then graudally, gradually, he came around.
And she will, too. At least we have vitamins now.
My own son is 4 and he's in a bit of a PBJ/pizza/pasta rut. Ah well.