293T safety, transfections with plasmid DNA
293T cells do incorporate partial virus genomes, consisting of about 4000 bases (about 11%, if I remember right) of the adenovirus type 5 genome (Cloning and Sequencing of the Cellular–Viral Junctions from the Human Adenovirus Type 5 Transformed 293 Cell Line). 

This amount of DNA is not enough to make a viable virus; it lacks all the structural components of the capsid, for example.

293T cells also include the SV40 Large T antigen, another viral gene. Again, this is not enough to make a replicating virus.

Therefore these cells don't pose any human health hazard aside from the normal almost entirely theoretical risk associated with any transformed human cell line (actually considerably less, since these were never tumor cells but were transformed in the lab, so they have no intrinsic adaptations to replication in humans, unlike say HeLa cells -- which are also demonstrably safe). 

293 cells have been continuously cultured in thousands or tens of thousands of labs for over 40 years, and I'm not aware of any safety incidents associated with them.  Of course, you should always handle cultured cells with appropriate precautions and protective wear, but these are not a specific concern.