Disclaimer: I am a biologist, if not working in virology or molecular biology. I actually read the paper.
The findings that the group reports are very interesting - based on the Slashdot headline you wouldn't believe it, but it's actually legit science.
However, there's a big caveat. This drug is a protein, which is a very large molecule. Almost all currently marketed drugs are small molecules, which is a huge difference to the body. You will never be able to administer this drug orally, as to the body it is "food" and will be degraded in the stomach. If you inject it, cells will not take it up and it will not be effective. This is why they attached sequences to the protein that make transduction (crossing of cell membranes into cells) possible. These sequences come from somewhat "dodgy" sources. One of them is actually a part of HIV. It is completely unclear how the body will react to that. The construct might, for example, trigger an intense immune response.
That said, I am delighted to see this kind of work published. Basically, people are reaping the fruits of decades of basic research in molecular biology to design drugs that can be "programmed" to do whatever you want. If the technical limitations can be overcome (that is, once proteins can be delivered to cells very specifically and non-invasively and once the cell killing mechanism can be made super specific), great innovations in medicine will become possible.