Looks like one of the bindweeds, likely field bindweed, Convolvulus arvensis. Whichever it is will be tough to control either with hoeing/pulling or herbicides because of their incredibly deep root systems. Also, seeds can remain viable in the soil for decades, so if you can prevent it from flowering it will limit the spread. Unyeilding tenacity is the only answer!
hortster

Here are some better pictures. I forgot to mention that I have not seen any type of flowers on this.

http://www.flickr.com/photos/33021253@N06/6872312870/in/photostream

http://www.flickr.com/photos/33021253@N06/6872312766/in/photostream/

I'll still go with bindweed. Indigenous to the entire USA and Canada. There's a reason!
hortster

Agreed, there are several types of vines that look almost exactly alike. Juvenile leaves can vary significantly from adult leaves, and flowers can be the same color as the leaves, rendering them virtually invisible.