There are such contraptions that merge the two, but most are large, many are a fold type wallet, and none are as versatile as need be. This is designed to hold anything including credit cards, business cards, cash, a notecard, or any combination of these.
You will need:
1.) A rigid or semi rigid plastic iphone case, ideally with a flat back. I used this one: http://www.incipio.com/cases/iphone-cases/iphone-4-4s-cases/iphone-4-4s-ngp-matte-semi-rigid-soft-shell-case.html, and it worked very nicely.
2.) 3/4" elastic strap (available at any sewing or hobby store)
3.) 1 1/2" wide aluminum sheet (1/16" thick)
4.) Band saw
5.) Utility or exacto knife
6.) Sewing machine or needle and thread
7.) A little bit of scrap fabric
8.) Fine grit sand paper (220)
Step 1: The Aluminum Sheet
To start, I measured the back of the phone case to see how long I could make it without interfering with the camera. Also, it had to be long enough to hold a business card. I settled on a length of 3 7/8" but you could make it a little longer or shorter.
Measure and cut the aluminum sheet to the desired length using a band saw with a metal cutting blade.
I did not want sharp corners, so I measured in 1/8" from each corner and cut off each at a diagonal.
In order for the aluminum to stay in place i notched it at the top and bottom of the long axis.
I measured in 5/16" from each side for the edge of the notch and cut the notch 1/16" deep, then used the band saw to remove the material.
Use sand paper to smooth all the edges and corners after the cuts are complete.