In response to some of the comments below: we've been using Cat5e exclusivley for analog installs for a few years now, and as Jon notes, have never run into an instance where power was an issue. Using mainly CNB VCM-24VF cameras with a whopping 2.2W draw (180mA @ 12V, 90mA @ 24V), we've even run two cameras, power and video, over 100' on a single line (this is not standard practice, of course, but it's what happens when the client wants to add another camera to an inaccessible location after construction is finished...). Keep in mind that these particular cameras have an operating voltage of something like 10-30V AC or DC, so almost any drop experienced with a 12V feed is inconsequential.
Second: no, DC power DOES NOT produce EMI, at least not on its own. Induced noise requires a varying voltage across the affected line; it's the changing field that creates the induced voltage. This is how transformers work... automotive spark coils... all manner of devices designed to take advantage of this effect. A constant, steady DC source means no varying field and thus no induced signal. A NOISY DC source may induce miniscule voltage on adjacent lines, but that's the fault of the noise itself.
Add to that, even with an AC source, not only will the twists in the signal line inherently reject noise, but the twists in the power pairs will cause cancellation of any field induced around those pairs as well. Add to that the balanced line created by using BALUNS (the term meaning "BALanaced/UNbalanced), which itself serves to cancel any received noise, and EMI is going to be pretty much a non-factor in this type of setup.
Third: keep in mind if you're planning for the potential to upgrade cameras later to IP, you're going to want to limit your runs to 100m anyway, unless you're going to use some form of extender... so while you CAN run a camera much farther, you may want to limit the length if planning for the future.
Fourth: Jon, you should check those BALUNs - passive video baluns are very simple devices, consisting of a matching transformer and a small capacitor or two; the guts of a typical video balun look (schematically) something like this:
The ones that include power typically just tie the power pairs together and run them straight through, meaning you can run DC *or* AC over them. Some designs also run the power over two pairs, and leave the fourth pair for things like audio, RS-485, etc.
Finally, I'd just like to throw in a little plug for a couple products we've used extensively in some past sites (NO affiliation with the source other than as a satisfied customer), and found them to work extremely well: a rackmount, combined balun-and-power unit, and a matching balun for the camera end. The "VPS" power units also combine each set of four channels into a single RJ-45, allowing easy running of four signals over a single UTP; on a couple of recent sites, we've terminated all our runs in patchbays in a wall rack, mounted the VPS boxes there, then put basic rackmount baluns in the rolling rack behind the DVR, and connected the DVR tails directly to them - this setup allows us to run 16 analog lines to the rack with only four UTP cables. Very efficient setup, and having the runs terminate in a patchbay makes switching over to IP cameras later as simple as unplugging from the VPS and plugging into a PoE switch; I haven't run 18/2 to ANY camera in at least three years.