No doubt that DT's has the science and technology to provide a very high quality product but.......if you spent about 10 minutes a day you too can grow a great phytoplankton like I've been doing for the past couple of years. I'll be the first to admit that my phytoplankton isn't as good as DT's but then it doesn't cost me an arm and a leg to feed my tank.
As far as contamination goes, IME as long as you don't over fertilize (which will kill your culture anyway) you should be fine. IME it is extremely difficult to poison your tank by overfeeding phyto. for experiemental purposes I've dumped a 2 litre bottle of phyto into my 29 gal and notice no changes in any params. I did have many filter feeding organisms in the tank (no sps) to consume it though.
I agree I would first purchase and read thoroughly the phytoplankton culture manual by Flordia aqua farms. before I began a home culture.
Also, if you want to deal with serious science and phytoplankton check out the university of texas phytoplankton dept.
here's a link to their site and many others.
http://www.bio.utexas.edu/research/utex/index.html
and others
http://www.botany.utoronto.ca/utcc/Links.html
Home culture is relatively easy and inexpensive, figure it this way (and I'm not knocking DT's at all, its a awesome product). for about 3 - 4 16oz bottles of DT's you can grow your own phytoplankton, probably more than one species, and feed your tank as often as you like, and give away extra to fellow reefers.
How can you go wrong?
One last thing, I wonder if Dennis can really explain the smell of DT's?
does anyone here who has grown their own phyto have that terrible smell to it? Mine doesn't even when it goes bad.
). I guess I will have to do some tests once Im up and running.
I was thinking of growing them a week and then fridging a week in a dark container so they dont get any light when I go to snack on something. That should mimic a vacation decently. I'll let you know - if you try it too we'll compare notes. 
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