Damn so much to cover...
Bikinihouse, in your last comment you're referring to an upsell after the member has already joined the site- that would not affect conversion ratios, as the members join had already gone through. Also, that upsell has another check box if you were interested in the offer and are not forced to take it.
Jimmy3Way, I do know the numbers and I do know that TopBucks is among many other companies who have seen positive results from adding this feature rather then negative ones. I don't care to share the numbers at this time because they are really not for public info...
It just tends to happen that unhappy people post more on threads like this than ones that are satisfied. Topbucks has been very consistent and will continue to be.
http://www.thestatistician.com/03.html
~~Alli