The simplest and most portable design I found is done from a business card. (This one requires some work 
)
Here is the link: http://www.instructables.com/id/Simple-b…Pod-stand/
My second choices:
BatRest iPhone folding stand I found on thinkgeek is listed as a iPhone 3G compatible but after talking to the customer support I was assured it works with iPhone 4.
I will update after I test it out myself. here is the link: http://www.thinkgeek.com/gadgets/cellphone/b6f5/
There are couple more choices but the two first choices I listed are the same size as a business card and will easily fit in my compact wallet.
other options:
http://www.myibend.com/
http://www.seskimo.com/index.php