Temporary Solution (by Ryuuga)
You Will Need:
1) You will need to borrow your friends iTunes account temporarily.
2) iTunes 8.0.1 or earlier. (DOES NOT work on iTunes 8.0.2)

The Problem: After choosing "None" as an option, it still requires you to enter a valid credit card number.

The Solution:
a) Login with an existing friends account and download a free application.
b) When you sync, you will get an error "computer not activated" or something similar. Don't worry.
c) Go to [Store] on the top bar and find for "activate computer" or something similar.
e) Sync the application again, sync will be in progress.
f) After syncing, let the app run on your iPhone for awhile.
g) Once satisfied, go back to [Store] and deactivate the computer (your friend can then change the password or something to prevent you from accessing).
*Your friend gets keep his iTunes account. However, when you sync with your iTunes, the free app will most likely be removed from your iPhone.