How to copy (backup) files from ESX3i?

You can use RCLI and vifs command. More info at
http://www.vm-help.com/esx/esx3i/esx_3i_rcli/vifs.php

Examples:
List files in datastore directory:
vifs --server 192.168.4.4 --username root --password ***** --dir "[Datastore1]/win2k3"

Download file from ESX3i Datastore:
vifs --server 192.168.4.4 --username root --dc ha-datacenter --password ***** --get "[Datastore1] /win2k3/win2k3-flat.vmdk" my-clone.vmdk

Notes:
File write speed around 1MBps only