Marketing Charts (great site) reports that Juniper Research is forecasting tablet sales to reach 81 million in 2015.
Sales in North America and Europe are expected to quadruple over this time frame. Tablet sales in Asia - currently negligible - are also expected to grow rapidly.
To put this into perspective, about 300 million PCs and 175 million Smartphones were sold globally in 2009.
The success of the iPad is leading to pretty much everyone who makes PCs and/or Smartphones to enter this market. Apple will soon have a lot of competition.