ESXi4.1已经改名叫做vSphere Hypervisor，前段时间将一台ESXi4.0 Update2 的Server升级到了ESXi4.1，过程概略如下：
升级工具：VMware vSphere Command-Line Interface(vSphere CLI)
步骤：
1.下载升级包（upgrade-from-ESXi4.0-to-4.1.0-xxxxxx-release.zip）；
2.关闭运行中的VM，并切换到maintenance mode；
3.开启CLI,并执行命令：
bin\vihostupdate.pl --server Server_IP -i -b 升级包路径名称 -B ESXi410-GA-esxupdate
bin\vihostupdate.pl --server Server_IP -i -b 升级包路径名称 -B ESXi410-GA
4.将Server重启并离开维护模式重启，完成升级过程；
5.升级Client。
附官方文档：
Upgrade an ESXi Host with the vihostupdate Utility
You can use the vihostupdate utility to upgrade from ESXi 4.0 to ESXi 4.1.
Prerequisites
Before you can upgrade an ESXi host from the command line, you must have access to a machine on which you can run the VMware vSphere Command-Line Interface (vSphere CLI). You can install the vSphere CLI on
a Microsoft Windows or Linux system or import the VMware vSphere Management Assistant (vMA) virtual appliance onto your ESXi host. For information about importing or installing the vSphere CLI, see the VMware
vSphere Command-Line Interface Installation and Reference Guide.
The target host must have 3GB of RAM if it is connected to vCenter Server.
For ESXi hosts, you must configure a scratch partition and reboot the host before proceeding with the upgrade.
You can configure a scratch partition for a host under the Software Advanced Settings in the Configuration tab of the vSphere Client.
Procedure
1 Download the following upgrade ZIP bundle from the VMware Web site to a location that is accessible
to the vSphere CLI machine.
upgrade-from-ESXi4.0-to-4.1.0-0.0.build#-release.zip
The upgrade ZIP bundle contains and esxupdate bulletin and an upgrade bulletin.
2 Power off any virtual machines that are running on the host and place the host into maintenance mode.
3 Install the esxupdate bulletin by running the following command on the vSphere CLI machine.
vihostupdate --server host name or IP address -i -b location of the ESXi upgrade ZIP bundle -
B ESXi410-GA-esxupdate
4 Install the upgrade bulletin by running the following command on the vSphere CLI machine.
vihostupdate --server host name or IP address -i -b location of the ESXi upgrade ZIP bundle -
B ESXi410-GA
5 Verify that the bulletins are installed on the ESXi host by running the following command.
vihostupdate.pl --server host name or IP address --query
6 Reboot the host.