As people rush out to begin migrating their pristine ESX 3.x and vSphere 4.0x environments to the brand new vSphere 4.1/vCenter 4.1 environment, it's important to keep in mind that there is an order to things. In some cases, products simply won't work together. For example, vCenter 4.0u2 is not compatible with ESX 4.1 As such, in order to ensure a smooth transition to vSphere 4.1, your first task should be to upgrade vCenter to the latest version and then begin the upgrade process on your individual ESX servers.
VMware keeps updated a document that outlines a number of various compatibility issues so that you can make better choices about what to upgrade first and what products work well together and which don't.
Before you consider significant changes to your VMware environment, run through this document to make sure you don't run into any nasty surprises along the way.