So how does one program such a computer?
In order to wrote a DNA program, one would of necessity have to be able to manipulate individual molecules within DNA in a reproducible way, and in huge enough quantities to allow for meaningful computations. And to be useful, the process would have to be rapid.
If we're able to do that, then the medical implications are enormous -- far eclipsing the impact of DNA computing. If we're not able to write programs, then DNA computing is unlikely ever to be more than a stunt.
On a side note, this development adds an interesting twist to the "origin of life" argument.