Why should Microsoft be able to talk up the benefits of Longhorn and to encourage migration to Longhorn-specific technology when they can't tell us when they are going to deliver?
Because it clues developers in on what's coming next, and because there's actual code to develop against. Longhorn and Whidbey alphas were released during the PDC. Yukon is currently at beta 1 (beta 2 is a few months away). Whidbey is supposed to start trickling further builds out to MSDN on a frequent basis in a few weeks. Longhorn will have a refresh build, then beta sometime around this summer. Though some of the APIs will change from the current build up to Beta, developers can at least experiment with the platform and plan out how their applications will integrate with the platform. Hardware developers can test the new driver models and plan new devices. If MS didn't give advanced notice, you'd have more ISVs/IHVs with incompatable products than you do now. It's bad enough when certain companies know a product is at Beta 3 or RC stage, and they don't start testing compatable drivers or a software update until a few months after RTM (if ever).
Additionally, the major thing that was reiterated at PDC and in other presentations wasn't that developers should start making the next version of their applications Longhorn-specific. It was that they should move to managed code and (preferably) build for the Limited user account. This would strengthen the security of their apps, possibly increase productivity, and prepare them for the move to Longhorn at a later date. By moving to managed code, not only could they target current platforms, but they could run their apps in Longhorn with few or no changes. Plus, it'd make it easier to add Longhorn-specific features if/when they choose to do so.
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