About a week ago, I started noticing holes in the side of the soil blocks. Not a whole lot, just one here, one there. This being my first experiment with soil blocks, I didn't know what to make of it. Then one day, I was re-arranging the soil blocks (yep, my soil blocks are passing the "can you pick it up?" test
) using a plastic fork to loosen it where it was stuck to the bottom of the tray, and (I must have accidentally stabbed it 
) a baby worm wriggled out! 
Like I said, I sifted the compost so I know it wasn't there before. There must be worm eggs hatching in the warmth of the seed starting area. I've found 4 more since then.
Aside from the holes that potentially weaken the blocks, I don't think they're doing any harm, and now that I'm looking for them, I'm finding castings among the blocks, so I figure I'm spared the trouble of giving fertilizer to the transplants as they grow. 
BTW, it's also my first year making home-made seed starting mix with unsterilized compost mix, and so far, it's working wonderfully. No damping off, no damaged transplants (except those very first trial of 3 brassica seedlings that got eaten by something) It hasn't happened since. I'm thinking it might have been a mouse in the garage, but I haven't had any more trouble ... possibly because I had my kitties thoroughly explore the garage several times....
I must've looked really happy because my 7 yr old said, "Mama, I guess they're your new pets." 
