As a novelty phone, it may probably be doable, but most likely as a wired phone, as cell phones are generally costly.

Now, what may be doable is to have the costly components of a cell phone as a removeable module, you insert into a less costly "skin", whose fasion may pass, or you may choose a different skin to match your purpose or fashion each day.

On the other hand a phone the size of a banana could be more then a phone. There are already 400MHz, 500Mhz PDA/Camera/internet/phones the size of a average calculator. I wouldn't mind a slightly bulkier phone if it did the same or more then a small desktop/laptop. About your idea though, I would have to go with the basic cell phone guts, with changable novelty skins.

If course, any long shape might suffice. A cucumber, a hot dog in a bun, even a vibrator - although it might ring at an inconvenient time.

I also thought of this idea long ago and have been working on building a prototype. I was told also that a removeable "skin" would be easiest and am working on building and patenting one. I also own the domain bananaphones.com

I would prefer solid plastic novelty cell phones rather than basic ones with removeable skins, but, we'll see.

Have you gone bananas?

I just googled this and found it's already been done. Here and here.

Check this out, http://www.makezine.com/blog/archive/2006/08/diy_banana_phone.htmlI think is great