I think you can use Dial on Demand Routing (DDR) as described here:
http://www.cisco.com/en/US/tech/tk801/tk133/tsd_technology_support_protocol_home.html
and this particular guide as well:
http://docwiki.cisco.com/wiki/Internetwork_Design_Guide_--_Designing_DDR_Internetworks#Dial_Backup_for_Leased_Lines
Basically your dial connection will not be active until certain traffic will pass through.
Now, to pass traffic only when primary is down, you can define a static floating route as explained above, which is a route with a higher admin distance than the default one, so if the normal route is up, nothing will pass to the dialer, if it's down, the route to the dialer will be used.
To detect that primary route is down, either use dynamic routing or route tracking as suggested.
I hope that helps.
Regards,
bastien.
