I think Dave nailed it here:
I think they occupy to similar a niche in the ecosystem to coexist very well.
When I read this thread, this is the first thing that came to mind. Strawberries and clover occupy a very similar root zone (though I believe clover will go deeper), as well as a spreading nature. Strawberries, as far as I know, are not intensive nitrogen users, and so would not necessarily benefit much from the association, besides living mulch (but this ends up quickly becoming too much of a good thing).
I have been trying to brainstorm a possible strawberry guild.
Here's what I thought might work, but I have not tried it anywhere.
Garlic, strawberries (June bearing), asparagus (seems to do alright in the acidic soils preferred by strawberries), and cabbage (will do well enough given enough moisture, and drainage, in almost any soil). My thoughts are a staggering of harvests, where one part of the guild bears then another, then another. The cabbages and the garlic would have similar harvest times. Cabbage provides nutrients (leaf mass) to the system. Dry garlic stalks, after curing, could be tossed back on the beds. New garlic cloves could be planted where the cabbage was, and also where strawberry plants have been moved from when they need to be cycled to new places. Asparagus, of course, comes up early in the spring, and is done it's prime production when the strawberries start kicking in.
Johnny Jump Up Violets might be a good cover crop. I don't know. They occupy a similar root zone, but I don't think they are quite as spreading as clover. I was thinking of the violets because they seem to enjoy nutrient rich but acid soil, just like strawberries.