Some people have been worried about past conviction and being refused entry to the US without a Visa. This link belongs to the US immigration dept and shows you what exactly the requirements are.
http://www.immigration.gov/lpBin/lp...?f=templates&fn=document-frame.htm#slb-act212
http://www.immigration.gov/lpBin/lp...?f=templates&fn=document-frame.htm#slb-act212