can anyone else tell me if that's definiately Oleg firmware?

read http://wl500g.info/showthread.php?t=1984

Limited samba support
Since 1.7.5.6-5 Samba 1.9.17p5 is included but, not enabled by default.
Important notice: the samba is totally unsupported, use on your own risk!

Small how-to:

* Be sure you've specified hostname in the LAN IP Setting (this name will used be by samba)
* add the following to your /usr/local/sbin/post-boot script):

/usr/sbin/smbd -D
/usr/sbin/nmbd -D

Once you've rebooted your wl500g should appear in workgroup called WORKGROUP and present you a share named "share". This share is read-only view of your usb drive (/tmp/harddisk/). There is also second invisible share called "share$" which is in the fact the read/write view of /tmp/harddisk/. No other access restriction present.

There are log files available for samba - /var/log/nmb and /var/log/smb, they could be examined if something does not work.
from http://wl500g.dyndns.org/