While this can be good, they need to get far smaller.
For example, some rodents are fast and very versatile, able to scramble through very small holes, and intelligent enough to figure out how to overcome obstacles.
Squirrel obstacle course:
http://www.youtube.com/watch?v=IxDKzHg_1e0
Remember that they can dig holes, have acute vision, smell and hearing, and respond very well to training.
They could likely be trained to give different responses to finding a living or dead person, important in a collapsed structure. And a small, lightweight camera and transmitter could even be put in their body, with only a fiber optic “lens” exposed.
I don’t know about squirrels in particular, but most rodents have fairly short lifespans. If training took even half as long as an intelligent SAR dog, the rodent’s entire youth and middle age would be eaten up already, and any genetic timebombs like a tendency towards cancer would be approaching quickly.
There’s already an issue with various types of working dogs living a relatively short time compared to their training period...I can’t see this being any better.