Ilovespinich - DS3 was exactly like this too, only wanted to eat bananas and he did grow out of it but it did take a while!
The only thing i would say is that i did find that he started to get tummy ache quite a bit and his poo's were really runny and smelly due to the fruit (sorry if tmi!) - i had to limit it to 2 bananas a day in the end because of tummy ache
I am always warned that bananas can be constipating for little people, but maybe that's because my toddler has severe poo issues sometimes and bananas are the only fruit he eats, and he seems to be more on an Atkins type diet at the mo, only wanting meat (and biscuits of course).
If there are not poo issues, I think bananas are better than a lot of foods!
Ds is only eating oranges and raisins at the mo, I still offer proper food but he usually refuses. Next week it will probably be cheese and bananas. We usually have a few weeks of nothing then he eats properly for a few weeks then back to just 1 or 2 things . Its very annoying!