Its hard to warranty the phones will never connect to the internet, and 100% sure they communicate with your phone service provider via their cellphone network. With a notebook you at least can remove the wireless and bluetooth cards.

For me a cheap notebook and pendrives are more recommended.

Sounds like a pretty secure idea and much less expensive than buying a computer to do nothing but store bit coin. Will one of these work without a sim card installed? 

If you have to send them, you would still have to connect to the internet. You will eventually connect the micro SD to a online computer one day and the data can be stolen. The microsd may have weak lifespan and gets damaged fast, there is always a risk about hardware dying.

Why not use a Raspberry Pi or similar? Cheap, small, and you don't have to worry about wifi.

If you just want to store the key then just get a cheap pda off of ebay for like a dollar.

I think it's a great idea but I would like to hear from people who disagree. I bought an ultra cheap android phone in china - $27 (ok I bought two so I can build a duplicate and send it to my brother as backup). No sim, powered up, reset to factory default, then removed apps that I didn't think I would need, games and messaging etc. Disabled wifi and BT, set up a pin to lock the phone.

I used my main everyday phone to backup the apps I wanted onto an SD card, probably just mycelium would be sufficient. Transferred the SD to the cheap phone, installed Mycelium and set up the pin to protect Mycelium. Write down the 15 character Mycelium passphrase. Then make a few test transfers and eventually send a few bigger sums (I haven't done this yet). Power off the phone, take the sd and move it to the backup phone. Go through the same process but restore the key from the first Mycelium wallet (I haven't done this step yet either). I'm probably going to use separate physical mail to send the pins and passphrase to my brother and ship the backup phone separately. But I'm still missing a few steps before I totally commit. I've had the phones for a few months and they power up ok so no issues with reliability yet. If it sounds like a good plan it would be great to build a step by step tutorial.

because backup file (from 2 years) don't work with the new client updated in the phone (since 1 hour) + private key are encoded ... (BIP something like that)

You can extract the private key and create and sign a raw transaction then broadcast it. The format would most likely still be accepted by the community. It is a good idea to stay in touch within the bitcoin community for updates.

smartphone sounds like a smarter plan to me than a laptop.. it's much smaller and easier to conceal.

Great idea actually, although you could always just put it onto a USB and put it somewhere safe in your house.