Apple probably wanted to focus 100% on software today. They seem to want to bring a lot of attention to iCloud. Even more than iOS 5 and Lion for now (see apple.com). Hardware announcements would have distracted people from iCloud, and Apple likes to be perceived as the company that democratizes everything. They had to make a big deal out of cloud computing like they invented it. Then people will continue to perceive them as innovators as other companies will inevitably follow the cloud hype right after.
They still need to refresh the Air, the white MacBook, the Mac mini, and possibly the Mac Pro as well as the Time Capsule / AirPorts. The first two are probably coming with the Back to School promotion, which Apple still has yet to announce. They're probably holding off for Lion. It would suck for a student to shell 1K+$ on a laptop to be then constantly reminded a month after that they should upgrade to Lion for 30$ through the App Store, which will probably filled with Lion ads. A lot of them are buying their first Mac and don't even know Lion is coming. Apple likes to have that ''full out of the box experience'', and don't want people to think they still don't have the full deal even after they paid for a premium computer.
I don't see them not making a Back to School promotion this year, students always have been an important part of the market for Apple, especially considering how loyal Apple customers are, once you go Mac you never go back, so they'd rather have them switch young. Plus they still have supplies of iPods to get rid of before the new ones come out. The promotion always started around the same date other years. There must be a reason why this year is different, doesn't it? Core 2 Duo must disappear, fast. Let's just wait a bit.