"The answer is of course subjective, varying by region and by individual defintions. To get into most private banks and wealth-management firms today, you need investible assets of at least $2 million - and most often, $10 million. In private-banking lingo, having $1 million counts as 'high net worth,' while it takes $25 million to $20 million to be 'ultra-high net worth.'"

The Rich-O-Meter
by Robert Frank
http://blogs.wsj.com/wealth/2007/02/01/the-rich-o-meter/

Also check out the Global Rich List, as mentioned in the article:
http://www.globalrichlist.com/