It is built in into 5.x also in the 5.0 Software of the devices.
I tried a few things, ( I only want to receive S/Mime mails ) but it did not work very good.
see here:
http://www.blackberryforums.com/bes-...light=S%2Fmime
In the components view,
open "email",
click on your server,
on the right side click on the "messaging" tab,
scroll down, there you can enable s/mime processing