Yes, but it's tricky right now. The delivery mechanisms are still rough. But for certain applications, in certain tissues, for certain kinds of diseases it is being done today (see Sangamo getting cleared for a gene therapy for hemophilia last week)[1].
Currently we have two ways to direct a genetic payload - one is localized (cellular) control of the actual delivery mechanism, the other is (biologically-relevant) temporal control of when the payload gets 'fired' or used.
Practically, we can strip some kinds of mammalian viruses of their own payload (and their ability to replicate), and utilize their own DNA insertion machinery (which is very nicely evolved to match our physiology) to insert a desired payload into the genome of a live organism. Some tissues are better at receiving specific payloads than others. (If you find a tissue and corresponding virus that delivers a tissue-specific viral infection, we can likely deliver a tissue-specific viral gene therapy to that tissue.)
Once the genetic payload is loaded into the genome we have other ways to control when the payload is actually turned on. We can make the payload sensitive to tissue-kind, or biological timing events, or sensitive to other kinds of biologically relevant environmental cues.
Both of these methods alone, however, are generally leaky. If the changes you are making are critical either for effect, or for preventing off-target effects, then these two methods alone are likely insufficient for a general population therapy to not be a dangerous gamble. The consequences of injecting the wrong code (or the right code to the wrong place) can be fairly catastrophic (induced, and immediate cancer). And that is precisely why a good 'text editor' is valuable.
One intervening point is that most viruses inject their payload into your genome randomly. And if they happen to overwrite some required genetic code of yours they cause cancer. Some viruses actually intentionally overwrite the code for your viral defense mechanisms (see HPV overwriting the P53 gene). In general, random injection of genetic payload into a live organism's genome (source code) is a bad idea.
The ability to have a good 'text editor' means we can now ensure that if the payload gets to the desired cells, then we can insert that payload into the genome in a desired location that is unlikely to cause problems. And since we can further regulate when the payload is fired, we now have 3 levels of control ( 1: localized cellular delivery, 2: localized genetic insertion, 3: biologically controlled activation). And those three levels together can significantly reduce the chance of mis-editing a live organism's genome.
Once these three regulatory mechanisms are made safe enough to not just be used in situations where risk/reward ratio is skewed by the lethality a disease, there is no reason they cannot also be used for an arbitrary biological effect.
[1] http://investor.sangamo.com/releasedetail.cfm?releaseid=9448...