if its a below ground pool and the tomato plants are near the water heater outflow then you have the heat of the water warming the earth making it nicer for the tomato plants. I would think they would have to be really close though.
http://www.builditsolar.com/Projects/Sunspace/ChicoHS/ChicoHS.htm

Here is a link that might be useful: chico hot springs greenhouse

Just as an aside, the DE that is used for pool filters is not meant for human consumption. I believe (but don't know) that it is chemically treated for use in pool filters. "Food grade" DE is sometimes used in the house and garden for pest control and is considered safe. I'm not sure I'd want the pool filter stuff dumped in my garden. But maybe I'm over-reacting.

Based on nothing other than observations I've made with pulling weeds where DE had been washed out before, I've noticed that the roots the grow in DE tend to be very large and numerous compared to those weeds I've pulled growing in regular soil.

I had read something a year or so ago that stated that when roots encounter sharp particles in their growing medium, they tend to branch more and DE is made up of very sharp particles (Diatoms) - especially the stuff used for pools. It's possible that the DE is helping to loosen the soil and, at the same time, help to form a more dense root system...

All of this is conjecture, of course.

Diatomaceous earth is an excellent soil additive.

They use that on high grade grass fields.
Very airy. very drainable, very good at holding moisture all at the same time.

I'm not s'prised your tomatoes are kicking butt over those neighbors of yours.

Using it as a filter is probably impregnating it with all sorts of goodness.

I was told at Home Depot that DE for pool filters is the same as for gardens. After I applied it, I found out differently.
Is it harmful? What do I need to be aware of?

From a site about DE:
"... Pool grade DE is Diatomaceous Earth produced for pool filters and it is treated with heat, causing the formerly amorphous silicon dioxide to assume crystalline form... Swimming pool DE ranges from 60% to 70% free silica..."

Basically, when they treat the DE for pool use, you now increase the crystalline silica content. If you Google crystalline silica, you will find there are some health risks, but all of them (that I could find) are related to breathing in the substance.

Crystalline silica is already a natural part of sand and most soils, so assuming the DE is mixed well with the soil, the odds of you breathing in free crystalline silica in significant quantities would seem to be minimal. When working with crystalline silica or products that may contain it, I would definitely wear protection for my eyes, skin and lungs, but if it's already integrated in your garden I would assume very little in the way of health impact.

I've included a link to OSHA's fact sheet about crystalline silica below.

Here is a link that might be useful: OSHA crystalline silica fact sheet

I'm going to agree with elskunkito. Also, check out the linked article.

Here is a link that might be useful: article