If you are the happy owner of ESX or ESXi 3.5 boxes, the time has come to apply a specific patch to your hosts in order to be able to continue updating these after June 1st, 2011.
The patch is not exactly the same for ESX or ESXi hosts.
For ESX apply patch ESX350-201012410-BG as indicated here. This patch for ESX hosts is a small one with a size of 11KB and it does not require you to reboot anything.
For ESXi, the situation is different. You have to install a bigger (238 MB) patch (found here) named ESXe350-201012401-I-BG and to do so all the VMs on the ESXi host must be shutdown or migrated to another host. A reboot of the ESXi host is also mandatory.
Again, this patch MUST be installed to continue patching hosts after June 1st, 2011.
Welcome to a virtual world with physical reboots.