When your clients should adopt Windows 8 depends on the kind of clients you serve. Chip Camden recommends testing the Release Preview and forming a rollout plan now.
Microsoft recently made the Windows 8 Release Preview available for download. We can probably expect to see Windows 8 released in time to start appearing on new systems before the end of the year. This means that unless you're in the enviable position of supporting only non-Microsoft systems, you'd better start thinking about how Windows 8 will affect your clients. If you haven't already, you should download the release preview and start playing with it to see what you can expect.
One question for which you should start seeking an answer is: when should your clients adopt Windows 8, if at all? Some businesses are still congratulating themselves for skipping Windows Vista and going directly from Windows XP to Windows 7. Windows 8 is on the wrong side of the every-other-release-sucks trend. While we could debate the accuracy of that trend (depending on how you number them), the historical percentage of problematic upgrades has nevertheless been about 50% at best. Hopefully, Microsoft won't continue that trend, but if it does, then it might be advisable to avoid Windows 8 altogether.
However, it also depends on what kind of clients you serve. End users can choose not to bother with an upgrade as long as they can find adequate support. My clients, though, are software vendors. If they choose not to support a new version of Windows, they're probably choosing to lose a significant number of customers. They must not only support each new version, they must also support it early -- ideally as soon as new systems begin shipping with it. Those clients should be looking at the release preview right now.
Problems to expect in Windows 8
For business users, application compatibility is probably the first concern. Each new version of Windows changes some aspect of application behavior, often in ways that Microsoft didn't foresee to be fatal or render the application unusable. Sometimes that's the programmer's fault -- relying on undocumented behavior after figuring things out experimentally, only to have that behavior pulled out from under them in the next version. Other times, Microsoft has changed documented behavior and forced developers to modify their code.
Windows 8 also includes a new version (4.5) of the .NET Framework. Steve Ives reports that he has already observed problems running existing .NET applications against this version. If your client is a software vendor, they'll need to plan for extensive testing under Windows 8 and the new framework before they claim to support it. End users will want to test the applications they use in the manner in which they use them before upgrading.
The hardware requirements for Windows 8 aren't much greater than for Windows 7, and recently purchased machines should qualify. Note though, that for applications that use the new Metro interface, they'll need at least 1024x768 resolution.
Speaking of Metro, users will need to get used to this new UI. Existing applications may still look like they did on previous versions of Windows, but launching them will be different. Users who have memorized keyboard shortcuts for navigating the Start Menu will have to learn new muscle memory, although a lot of the other shortcuts remain the same.
Software vendors who intend to support Windows 8 should plan to rethink their user experience. They should evaluate whether the Metro interface would be useful and/or desirable. If so, they're looking at a complete redesign and rewrite of the UI, or at least the portions that need to go to Metro. At minimum, they should revisit all of their Start Menu entries to determine how best to make these options available to end users.
Test and plan now
Unlike the transition from Vista to Windows 7, Windows 8 changes the user experience and the application environment in significant ways. Don't just assume that everything's going to work. Start testing with the release preview, and form a plan for how you're going to roll out any changes that you'll need. Discuss these in detail with your clients before they (or their customers) start buying new systems with Windows 8 already installed.
Windows 8 Release Preview screenshot featured was taken by TechRepublic's Mark Kaelin.