After a little google-ing I found a Red Hat Enterprise password reset procedure, that also works on ESX 3, actually it's quite easy.
- Reboot your ESX-server.
- In the Linux GRUB menu select "Service Console" and press "e" to edit.
- Go to the line that starts with "kernel" and press "e" again
- Append the word "single" at the end of the line and press enter
- Press "b" to boot with these options
- Once the system is booted you get a Linux command prompt, where you can enter "passwd" to change the password
- Reboot and login using your new password