9to5mac reports rumors that Apple plans to replace the iTunes app with a Safari-only web app.
Before commenting on how credible this might be, let me suggest a little experiment. Fire up iTunes and point it to the iPhone/iPod/iPad app store, also fire up the Mac app store application, then do a few parallel searches for apps that exist on both platforms. Note how the performance of the iTunes app compares with that of the Mac app store application.
Apple has been building out a web version of iTunes for some time. Take the Twitter app, for instance. If you search for Twitter, in the iOS app store using the iTunes app, and click on the bird, nothing will happen because you're already there, but if you right-click (control-click) on the bird and choose copy link, then paste that link into Safari and activate it, instead of being taken back to the iTunes app you're instead taken to a look-alike web page.
Given that Apple has a contractual obligation to constrain the installation of non-free apps to devices owned by people who've paid for those apps, it makes sense that they would limit access to a web version of the iTunes store to Safari, which they can control, possibly using a plug-in for this purpose. They might also use Safari only to conduct transactions with the store and use a separate program to manage the configuration of various devices.
Whatever the details, there's probably some truth to this rumor.