I've been contemplating sending my Canon 40D + lenses in for cleaning & maintenance, and as luck would have it, one of the answers on a recent question recommends periodic maintenance to prolong the service life of a camera. As you can imagine, googling this turned up a range of opinions ranging from "if it ain't broke, don't fix it" to "service religiously every x months or y shots".
I'd like to get a better handle on the specific benefits of preventative service for DSLRs, including what, in addition to cleaning, is typically done during servicing. If you have any specific before & after experience with this sort of service ("they fixed stuff I didn't even know was broken" or "I swear I didn't have that problem before I sent my camera in"), that's great, too.
For purposes of this question, I'm talking about a Canon body, but I'd expect experiences with other brands to be pretty equivalent. I'd also expect similar experiences with servicing lenses to be relevant, especially if there's any benefit to sending lenses in with bodies.