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keep in mind that Tiger was the latest OS for what seems a larger period of time as Apple released two versions of Tiger in that period, PPC & Intel. Leopard's period of time being the latest OS may be shorter than Tiger's
 
Considering they haven't even announced an upcoming OS yet, I don't think whatever's next will arrive for another two years. Relax and wait for the service packs. :^)
 
It has been about 6 to 32 months, so far its getting slower and slower in upgrade, which is reasonable consider the OS is getting more and more stabilized in terms of how much change can be further made to warrant an upgrade.
 
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