Current technology

One available method is gene manipulation through virus insertion. The main mechanism by which this operates is a fairly standard one; one is taking advantage of the way viruses hi-jack the cellular machinery to make multiple copies of themselves. The way they do this is by inserting a segment of their genome into the DNA of the host after penetrating the cellular wall, and so use this to instruct the host cell to produce multiple copies of the virus. This process could be utilized as a mechanism to spread new DNA by altering the genome of the virus itself to include the desired new DNA sequence. The altered virus could be injected into a host cell, where it would propagate and eventually spread the new DNA throughout the host’s body. Problems with this approach may be:
Randomness of the insertion of the DNA fragment wanted (this might lead, for instance, to a cell turning cancerous, or being more subtly disrupted),
Possible lack of control over the immune response to the "retrovirus", and
Possible problems getting complete transformation of the target cell population.
Another approach is to take advantage of the cell's endogenous ability to undergo homologous recombination wherein an exogenous DNA fragement with ends matching an endogenous gene replaces the target gene by action of enzymes called recombinases. This is the technology used for "knockouts" and "knock-ins" and generally cannot be performed on an intact organism and is therefore only a candidate for germline (i.e. single cell) genetic manipulation. This technology has very widespread usage in mouse genetics wherein genetically altered embryos are grown in a surrogate mother. This could easily be extended to humans in the very near future as there are no major additional technological hurdles remaining to engineer humans versus mice (though minor hurdles certainly remain). The primary hurdle is ethical, as the efficiency of this process can be quite poor and many embryos would be sacrificed in order to produce one genetically modified offspring.