These pansies started blooming randomly in my yard within the past week.
<a href="http://www.flickr.com/photos/8007861@N04/5514197722/" title="Naturalized pansies by mtbikernate, on Flickr"><img src="http://farm6.static.flickr.com/5059/5514197722_ba7bb9dafe.jpg" width="500" height="375" alt="Naturalized pansies" /></a>
Photo by my wife
To be honest, I'm not strongly motivated to remove them at this point. If they are hardy enough to seed themselves randomly in my yard (I sure didn't plant them) and they're not listed as invasive, I welcome the color from the flowers and the greenness from the leaves in a portion of my yard that doesn't grow many things well.
Since I'm not apt to spread chemicals or broadcast water all over the place, I am not going to grow a "perfect" manicured lawn. What lawn space I do have is going to be filled with a wide variety of weeds and such. I will fight battles worth fighting and I've got a lot of projects on my plate for the year.