We mentioned the new Store Setting tab just a short time ago. daveizzle on Twitter made the observation that this functionality makes it seem like you could theoretically sign into one account, download apps, sign out, sign into another account, and download apps from that account as well, all onto the same iPhone or iPod touch. Boom. Support for multiple iTunes accounts right on the device.
Why would that be interesting? Say your spouse had already bought a $9.99 app. App Store lets the same account re-download that app to the iPhone without a second charge, so it becomes a way to "share" applications (as long as you trust the person enough to put your iTunes password into their device). Another scenario would be a UK users wanting to download a US-only app. They could log into a US account, download the region-restricted app, then log out and log back into their own, local account.
Note: This has been possible all along via syncing from iTunes. Any apps downloaded from any account would transfer over to the iPhone. This merely extends the purchase and/or download process to the iPhone, further releasing iTunes tethered hold.
(Of course, since you can't re-download music for free, and you can't download movie or TV show videos directly to the iPhone, this pertains only to App Store content -- likely to prevent Big Media heads from 'sploding).