We've mentioned previously how iPhone 3.0 Beta 4 enabled an App Store tab in Preferences for managing and switching between iTunes Accounts. Looks like this feature is also available directly from the App Store application itself.
Scroll down to the bottom of a main page (i.e. before you've selected a specific app to look at) and you'll find a button containing your account user ID.
Tap on the button and you'll be given options to View Account, Sign Out, or Cancel.
Much more convenient than having to exit, launch Settings, navigate, switch accounts, go back...
Thanks Apple! More screenshots after the break...
(And thanks Roberto for the tip!)