According to VMware knowledge base article 1317898 it is not possible to reset the root password of an ESXi installation. Well, maybe it's not easy for non-Unix guys, but it certainly is possible.
The files containing the password hashes is called "shadow". It is contained in a nested structure of archives:
You cannot read the password of the root account, as it is one-way-hashed, but you can replace it with a known one or remove it altogether.
Here's a step-by-step guide how change the password file:
tar xzf /mnt/Hypervisor3/state.tgz
tar xzf local.tgz
Probably the easiest way is to replace the line starting with "root" with the line of another user account of which you know the password. You only need to change the name at the beginning of the line (before the first colon) to "root". You've then set the root password to a password you know.
Alternatively, you can just remove the hash altogether (everything between the first and the second colon) and login to the service console as root with no password at all.
mv /mnt/Hypervisor3/state.tgz /mnt/Hypervisor3/state.tgz.bak
tar czf local.tgz etc
tar czf state.tgz local.tgz
mv state.tgz /mnt/Hypervisor3/
Thanks Rob for testing it with ESXi 5.0 and letting me know.