Welcome to vSphere-land!

vSphere Upgrade Center - Advisor (VMware)
vSphere Upgrade Center - Overview (VMware)
vSphere Upgrade Center - Resources (VMware)
vSphere Pre-requisites Checklist (VMware)
vSphere 4.0 Software Compatibility Matrix (VMware)
Upgrading to VMware vSphere: Test first, deploy later (SearchVMware.com)
Various methods for upgrading to vSphere (SearchVMware.com)
vSphere Upgrade Center Blog (VMware)
The Path to vSphere Unleashed Pt 1 - VMware VirtualCenter Management Components (VMware)
The Path to vSphere Unleashed Pt 2 - VMware ESX Host Migration Methods (VMware)
The Path to vSphere Unleashed Pt 3 - Virtual Machine Upgrades (VMware Tools and Virtual Hardware) (VMware)
The Path to vSphere Unleashed Pt 4 - Implementing VMware vSphere Licensing (VMware)
How to Upgrade from VMware Infrastructure 3 to VMware vSphere 4 (VMware)
Remote in-place upgrade from ESX3.5 to vSphere (Vinf.net)
vSphere Virtual Machine Upgrade Process (Scott Lowe)
Upgrade VMware vCenter 2.5 to 4.0 Video (VMware Tips)
vSphere upgrade experience, day 1 (Virtualization Evangelist)
vSphere - Upgrade and Best Practices (Technodrone)
Adding ESX 3.5 Servers to vCenter 4.0 (Technodrone)
Understanding the Upgrade Process (Technodrone)
vCenter 4 Upgrade A Success! (Virtual-Jay)
Performing an offline upgrade from ESX 3.x to ESX 4.0 on a local ESX host (KB Article)
Upgrading to ESX 4.0 and vCenter 4.0 best practices (KB Article)
Upgrading an ESX 3.x virtual machine to ESX 4.0 (KB Article)
Upgrading ESXi 3.5 hosts with OEM server vendor’s specific components to ESXi 4.0 (KB Article)
How to create default alarms when upgrading to vSphere 4.0 (KB Article)
When you use VMware vSphere to perform an ESX host upgrade, the upgrade might fail (KB Article)
Sometimes the upgrade from ESXi 3.5 to ESXi 4.0 fails when the host is trying to reboot (KB Article)
Performing an offline upgrade from ESX 3.x to ESX 4.0 on a local ESX host (KB Article)
VMotion stops working after upgrading to vSphere 4.0 with a CPU of the host is incompatible error (KB Article)
VMotion stops working with a Licensing server is not configured error after upgrading to vSphere 4 (KB Article)