Hi
You can create a IPSEC tunnel over the DSL backup link between the locations when the primary connection (leased line) goes down.
You need to do the necessary IPSEC configs and apply the crypto map under the dialer interface which you use to dial out for the DSL Connection.
Pls do take care of encrypting the required interesting traffic rather encrypting all default traffic passing through the dialer interface(DSL link).
once you are done with the ipsec config do create a static route with some metric attached to it
This route will act as a secondary route and will come into act once the primary route through the leased line goes off when the leased line goes down.
if you need more info on the same do post out your current config here..
regds