I don't know of a Cerberus alternative, but I'd just take the obvious security measures you can, put a lock on the phone with an auto timeout, I don't know what version of Symbian offers encryption but I've got a Nokia 700 as a backup phone running Belle FP2 and it offers encryption of the phone memory and SD card, you can also setup remote locking.
Settings > Phone > Phone management > Security settings > Phone and SIM card > Remote phone locking
You can also set it to auto lock should the SIM card be changed in the same place.
I'll have a poke around but it's not very well supported understandably now and as the other guys have enjoyed poking fun you can probably see why, although I love the 700 myself, cracking little mobile,
J.
EDIT: You can get Kaspersky
http://www.kaspersky.co.uk/kaspersky_mobile_security, it is Free in the Nokia Store but says £14.99 on the website, maybe it is just a trial from the Store, I don't know but supports remote locate and wipe...