Greetings Jeff:
Yes, there are some preemergent herbicides that are labeled for tree planting practices.
Some of the common preemergent herbicides I am familiar with include:
Pendulum (many annual grasses and certain broadleaf weeds)
Oust (pre- and post emergent; some annual and biannual grasses and broadleaf weeds)
Simazine (many annual grasses and certain broadleaf weeds)
You are required by law to follow the directions on the label of these herbicides. The aforementioned herbicides 'may' be applied directly over the top of “dormant” trees, but not over the top of “leafed-out” trees. Again, it is very important that you follow the directions and rates listed on the herbicide label. Some of the aforementioned herbicides are not labeled for certain tree species.
Here is a good resource from Wisconsin DNR:
http://dnr.wi.gov/forestry/Fh/PDF/HerbicidesRegisteredforTreePlanting2006.pdf