Looks a good deal like a redbud seedling. Can you post a closeup of the foliage?
hortster

It is a dark and stormy night at the moment =:-o but I will try and get another picture tomorrow. The leaves are round, rather like a water lily.

I have taken 2 more pictures this morning - one of a single leaf so you can see the size, shape & colour, and the other of a branch, which shows that the leaves are single, alternately spaced, and that the stem is smooth, reddish-brown in colour with paler flecks. (Now all I have to do is try and work out how to upload the pictures!!)

Here is a redbud seedling behind my garage. The leaves are more heart shaped although I suppose there can be different varieties. I'm sure mine is a redbud, think it's called an Eastern Redbud even though they are prolific in the midwest.

Here are some seedlings I started in May of 2007 but lost them all the following winter. I picked the pods off a neighbor's tree, cracked out the seeds, dried them, and sandpapered them to start faster. Somebody on a forum here told me to do that.

Sorry I don't know what you have there. My first thought was redbud, too.

another pix from OP
http://forums.gardenweb.com/forums/load/weeds/msg08070247482.html

Here is a link that might be useful: another pix

Looks like a Redbud to me too!

I have a large Redbud in our yard and it seeds all over the place. It's a great tree, maybe you could dig it out and transplant it somewhere.