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.
Step 2: Prep the Case
I used a drill with a 7/32" bit to make a whole at each end of the notch. The plastic was a little too soft and the whole did not help. I would suggest just going straight for the knife unless your case is a hard plastic.
Use a straight edge to guide, and make two cuts with a knife connecting the marks you just made at the edge of each notch. The slits only need to be wide enough for the elastic to fit through.
Once the slits are cut, feed the elastic though each slit and cut it long enough so that it easily wraps around the outside of the case with the aluminum and overlaps inside the case.
Check the aluminum and elastic for fit.
Step 3: Sewing the Elastic
Remove the aluminum and feed the elastic through each of the slits. Feed the elastic so the ends are sticking out the backside of the case.
Cut a small piece of scrap fabric. This will be used to help join the ends without the ends overlapping to make the joint as this as possible.
Place the scrap fabric under one end, wrap it over the top and put a few stitches through the elastic and the scrap to hold it in place. Stretch the elastic to get as much free material as possible so you can sew it. Put a few stitches through the other end of the elastic and the scrap as well. make sure the length is correct and that the ends are butted up against each other between the scrap fabric.
If you're using a sewing machine, use a very wide stitch pattern and sew across the joint where the ends meet.
Once this is completed, and the joint is sewn to your satisfaction, feed it through the slits so that the joint is on the inside of the phone case.
Step 4: Finish and Minimize
Prepare for compliments on you sleek new wallet. This is great for nights out when you don't need anything but a card, your ID, and maybe some cash. I also love it for keeping a to-do list or grocery list handy. Enjoy!