I had a grass lawn put in last fall after solarizing a north facing bed of English ivy. This spring these largish mottled leaves started appearing in my lawn. I have no idea what they are since they don't appear in any lists of common broadleaf weeds. Some of these plants are starting to flower and it looks like it might be yellowish colored.
That's not a weed, it's a shade-loving wildflower, Erythronium. If you're lucky, you usually find them in deciduous woodlands. My guess is that either your house is fairly new and was built where woods once were, or there used to be a shade garden where your lawn is.
There are several American species, but if the cut-off yellow bud is from one of these plants, then it's possibly Erythronium americanum. The leaves, though, look more like the E. albidum species. Here are some photos of the blooms:
E. americanum: http://keywordsuggest.org/gallery/415143.html E. albidum: http://pics.davesgarden.com/pics/2008/05/14/trioadastra/3f9d68.jpg
There are other native species, as well as Asian species. The two I listed are more common in the East and Midwest US.
The Common name is Dogtooth Violet, but it's actually a lily. This is a wonderful plant - a spring ephemeral, which means that it goes dormant after flowering. If you have some deciduous shade, I'd move them there (hopefully in a couple of weeks or so, and before a good rain). They will go dormant immediately. Next year, they'll sprout again and probably bloom for you. It's best if you can avoid mowing the plants again until you move them. If you don't want to move them, then do nothing - they'll disappear on their own in a month or two.