iPhone OS 3.0 Beta 4 now supports the use of multiple iTunes accounts, according to the Apple Blog.
There is now a new “Sign In” button at the bottom of the Featured page in App Store. Apparently, you’ll be able to create a new iTunes account, though this does not seem to be working at the moment.
In Settings, the “Store” entry that was a placeholder in Beta 3, and which did nothing, has gained an icon and is now working. Tapping on it brings you to a page where you can sign in and out of an account or view account information. If you choose “View Account”, you’ll be prompted to sign in once more, just like how it is when viewing account information in iTunes on the desktop.
Once you’ve done so, you’ll be presented with information such as your contact details and the balance remaining in your account. You will also be able to redeem gift codes from within this page, instead of having to find the “Redeem” button in the Downloads section of the built-in iTunes app, a step I have always felt to be rather counterintuitive.
This feature may have been added to work in conjunction with the In-App purchases supported by OS 3.0.