I used this to upgrade an ESX host from 3.0.1 to 3.0.2. It is likely that the same procedure can be used with future releases. Use caution when upgrading production servers as this procedure DOES take the host offline.

From http://www.vmware.com/pdf/vi3_installation_guide.pdf

To upgrade using esxupdate

1. Download the esxupdate tar archive bundle.
2. Extract the compressed tar archive (gtar xvzf file.tar.gz) and change to the newly created directory.
3. Type esxupdate -n update.
4. When the update command completes, reboot the ESX Server host.