A lot of homes are at least partially wired. The "phone line" cable in homes built in the last decade or so is almost certainly cat 5.
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1. The other irritating thing was discovering that the light switches were attached only to the faceplate which in turn was attached only to the drywall.
Good call. Although, I just had it done recently and I was pleasantly surprised at how noninvasive it was (the electricians worked really hard, so that's part of it). We moved things out of the way for where we wanted the jacks installed and they ran the cables through the wall with a minimal amount of holes. Definitely would have been even easier if we didn't have anything in the house at all, but there was way less ceremony on my part than I was expecting.