Even if Google didn't mention it in the release notes, Chrome 7 has support for installing web apps, but Chrome Web Store is not yet released. The store will be the most important new feature in Chrome 8, which will be launched very soon.
"Chrome 8 is the first version that supports the Chrome Web Store," mentions a Google Chrome engineer. There's already a new icon for the store in Chromium and this should be the final icon.
Google Chrome 7 was launched on October 21, almost 6 weeks ago, so Chrome 8 and Chrome Web Store should be available this week.