Preview at WWDC likely in June, followed by September launch.
Apple to Sell Software via Software Update?
The Mac OS X 10.3 (Panther) Software Update's English.lproj Localizable.strings file includes the following messages (which were not present in Mac OS X 10.2):
A 1-Click account is required to purchase software.
To purchase software, you need an Apple account with 1-Click ordering enabled. If you don't have an Apple account, you can create one in just a few minutes by clicking Set Up.
Save ?? with a .Mac membership.
.Mac members enjoy free or discounted pricing on software. Click Set Up to make sure your member name and password are correct, or sign up for a .Mac membership. Click Continue if you don't want a .Mac membership now.
Longstanding rumors have hinted that Apple will be revamping Software Update to allow 3rd party developers to provide their own updates through the Apple utility. It's unclear if purchasable software would be limited to Apple's own selections or opened to 3rd party developers as well.