Just keep in mind that certain changes from older versions should be identified before upgrading to enable a smooth transition. These are just some pointers for what should be thought about from the outset.
As with most new versions of software, there are always simple things that might cause unnecessary pain. The main thing to point out is that if you upgrade your ESX or ESXi hosts to vSphere 4.1 first then you won’t be able to add them to vCenter 4.0.x or older so the best process would be to upgrade vCenter first to 4.1 as this is compatible and supports the current latest version of ESX and ESXi hosts along with the older versions.
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