In our recent Research Note, "Ten Reasons You Should and Shouldn't Care About Microsoft's Windows Vista Client," we concluded that Windows 2000 users should plan to begin migrations beginning in early 2008 and most Windows XP users should pursue a strategy of managed diversity, bringing in Windows Vista on new machines starting in 2008.

These recommendations have been misinterpreted in some instances and have led to several client questions. We are not saying to ignore Windows Vista until 2008 — we're saying to be ready to deploy it in 2008. To be able to deploy Windows Vista in 2008, a significant amount of work must be done to prepare. Deployments don't just happen overnight. It will take many organizations about 18 months from the time Windows Vista ships to test applications, get independent software vendors (ISVs) to support applications, build images and run pilots. Organizations that ignore Vista until 2008 will not be ready to deploy it until 2010.

The biggest issue is application compatibility. There are two issues here:

Will my applications run on Windows Vista?
Will my application vendors support their applications running on Windows Vista?
For a mission-critical application, even if the answer to No. 1 is "yes," a "no" answer to No. 2 may be enough to delay deployment. Vendor support for new client operating systems (OSs) doesn't happen overnight either. In 2003 clients told that they had just received new versions of certain vertical business applications that were finally supported by the vendor on Windows 2000, a 3-year-old, one-generation-behind OS. Similar delays will likely be common for vendors supporting their applications on Windows Vista.

Add to this the complication of organizations having hundreds or thousands of applications to test (we estimate that for organizations with 5,000 to 50,000 PCs, dividing their number of users by 10 can give a good indication of how many applications might need testing) and you can see that the timeline required for preparation is quite long.
 

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Cette réponse du Gartner fait suite à l'article publié sur http://news.zdnet.co.uk/software/windows/0,39020396,39236741,00.htm concernant Gartner: Ignore Vista until 2008 qui a fait beaucoup de bruit sur le web ces jours-ci.