Oh wait. Why not just buy a mobile phone for $20 that has an SD slot? Then you can do what you asked!
Can a Phone read "normal" SDcard? No, usually smartphones can only read microSD card, so here's why I'm trying to design this kind of device.
I was thinking about a person in vacation, in his hotel room, connected to the hotel WiFi network with his smartphone. He has taken a lot of photos and wants to upload them directly from his room, without using a laptop. He could connect this device to the smartphone, then browse the file system in the SD card of his camera, then select and upload the best photos and finally format the card to get more space for more photos or video.
Start with reading the SD card?
Yes that would be useful to create first a standard interface for "normal" SDcards file browsing over a smartphone. Then the interface could be used for other systems. The objective is to create this kinda standard interface for a mobile phone to browse external flash memories, then the applications would only depend on the smartphone's software. So you're right, maybe the card reader is the best thing to do first.
Any advice for the best mobile operating system to develop for? Would it be hard on iOS?