Thank you for posting.
The Instant Web Publishing reproduces as much of the client functionality as possible. As you noted, it does enable you to input data and view layouts. Since a "report" is generally just a specialized layout, it should be possible to view one in the Instant Web Publishing. For a full list of differences between using the Instant Web Publishing and the client, please refer to page 25 of the Instant Web Publishing Guide:
The key point from this list would be: "Web users cannot view and print complex, multi-page reports." Simple, single page reports, however, should be possible.