You might want to take a look at the responses to a similar question just a bit down the page.

http://forums.gardenweb.com/discussions/4686464/planting-kolorabi-near-tomatoes?n=3

IMO the likely hood that it is the kale causing problems for your tomatoes is far less likely than just simple over-crowding being the cause. In other words there are several possible causes for stunted tomato plants - nutrient deficiency, shade, root rot, viral diseases, nematodes, herbicide damage, soil compaction, etc. to name just a few. Stunting by Brassicas would be way down on that list so you would want to rule out all the other possible causes first.

Dave

Thanks Dave, it's probable that they are overcrowed by the kale, as it is getting pretty big. Appreciate your insight.

Greg

i don't know how big the kale plants are, as there are both big and small kale plants to be sure it's over crowding (you be the judge).. but my guess is soil fertility. might be good enough for the kale, but maybe not sufficient for the tomato (ie different feeding requirements)..
even things like having slightly more compost in one section compared to another can have huge impact in plant growth. an example would be some of my tomato plants which some areas i dumped a bit more compost and those plants (identical variety) grew noticeably bigger than another plant several feet from it.

Good point Steve - kale is such a nitrogen hog it could easily be stealing needed nutrients from the tomato plants. One reason why inter-mixing crops with different nutrient and water needs often leads to failure to one or the other.

Dave

Since Kale is a cold-weather plant, you might consider the idea that it probably utilizes phosphorus in the soil very well (since phosphorus is less available when it's cold, and phosphorus helps plants with cold-tolerance; as it's more available when it's hot the plant might be sucking it up quickly). If the soil doesn't have enough phosphorus, your tomato plants would probably be stunted (phosphorus helps with plant maturity, leaf size, flowers, roots and such). Do you see any purple on the undersides of the tomato leaves? If so, that can be a sign of phosphorus deficiency. If not, the deficiency could still be present, but probably not as bad. Anyway, I don't know if this is your situation, but it's some food for thought, at least. :)