I believe glyphosate (the main ingredient of roundup and many other weed killers) is quite safe when used properly. There have been extensive studies on it over the years which have proven it to have very low toxicity.
Take a look at this link:

http://ace.ace.orst.edu/info/extoxnet/pips/glyphosa.htm

Overall, there is no major hazard from direct ingestion and it does not build up in the body like other toxic substances (e.g. lead). It also breaks down easily in quite little time in both soil and water, thus it's not building up in the ground where it could dammage new plants and filter in to our food.

I rarely use weed killer on principal that it's expencive and most weeds can be pulled out and forgotten. However, for persistent weeds that I would be pulling up in to next decade without weed killer (namely my battle with bindweed infestation), I quite happily use the glyphosate. I can't say how safe it is when used commercially, but I occasionally use it in my own garden to get the job done faster and easier, and knowing it's not going to poison me or my pets and that it will break down without trace in about 4 weeks. Thus, I think it's fine to use it at home because fusarium fungus isn't going to matter in my or most anybody else's garden. Also, most people don't use it in the large unselective ammounts at home which cause the problems that might happen to crop foods. No doubt these issues if correct will mean a new generation of glyphosate based weed killers that have fungicide in them to kill the mould. If it's currently safe on farm crops I don't know, I guess I'll leave that to the scientists and just worry about my own garden for now...