I have done this sort of thing for customers and seen numerous other cases where it was done this way. The two items that leap most to mind for me.
The first is to make sure that the backup solution is up all the time so the links can be monitored. Murphy's Law being what it is, the backup link you need will be the one that is down if you aren't monitoring it all the time.
The second thing is that longest match is the first thing in selecting an IP route. That means it is critical that you advertise the networks at the same prefix lengths in both routing protocols, or you could have traffic going to the longest match over your backup links when that isn't the preferred path. If you run EIGRP in your core, and BGP on the MPLS WAN (a very common scenario), one way to do this is to use a different EIGRP ASN on the DMVPN links. You can then redistribute into the core EIGRP ASN and feel good that those routes will be less preferred because they are external. The key point in here is if you use summaries (which is good practice when you can), may sure you send summaries of the same length into both routing protocols to avoid longest match causing sub-optimal traffic patterns. You can apply the same sort of ideas to different IGP's in the core and backup. You could even have the backup path just send a really broad summary into the core. Then you don't have to worry about longest match causing issues.
