A a citizen, I filled up a I-130 form for my parents; they live abroad, but they already have a tourist visa; may they visit USA for short time while they are waiting for the GC process, and then go back to their country, keeping back and forth in the meantime?
The law is uncertain. This type of entry will be left at the discretion of CBP. I have a blog entry on this subject. Please review it: http://forums.immigration.com/blog.php?u=1