It could use an apt repository in the back-end.
So developers would submit their applications and users could browse(through categories), view, read about, comment, download, etc, all the applications.
It would also make third-party applications always up-to-date, since the developers would have to submit updates.
I own an iPod Touch and the Apple's App Store is very nice.
Google's Android have his own "Android Market" and even Microsoft launched his "Windows Marketplace".
I'ts great idea... Ubuntu should be narrow orientated to open source software, why limit ourself... we should have choice about which software we'll use, paid or open source. Also this will improve user experience for user switched from macos & windows