I've posted links to a few other studies in here. Feel free todo research on your own. Seriously - I'm interested in learning more about this. Thus far the overwhelming percentage of studies point toward long term problems with spanking.
Have you considered the social pretense under which those studies are conducted? As I pointed out in an earlier post, in England and the United States, corporal punishment is the status quo. Research is often done in the name of promoting change to that status quo or challenging existing beliefs.
A better place to look would probably be studies done by Scandanavian researchers to challenge their current policies of no corporal punishment. Though there are other factors to consider here as well, seeing as how their ideas about education differs drastically from that of the US as well as their socioeconomic demographic.
The short of it is that it's a difficult conclusion to draw from "studies"... and for people that were raised using corporal punishment, many don't seem to object to it (I believe this was a consisitent theme from most studies) In that context, someone's personal experience will likely trump anything they read from a book or research paper.