I found this rather unusual weed in a lawn on July 9, 2009 (the picture on the bottom). I also found the same species on campus last year (picture on the top). It is unusual for this area. Post your guess as to what it is in the comment section. The answer will follow in a couple of days.
Congratulations to Nick Dunlap for figuring this one out.
It is Yellow Rocket (Barbaria vulgaris), part of the mustard family. It was quite common in Ohio when I was there many years ago, but I just began seeing it three years ago in Iowa. Since then, I have seen a number of them in central Iowa turf. It is not hard to control, similar to dandelion.