According to this article at ePaynews.com,
"Self-service and self-checkout is growing in multiple markets, with Frost & Sullivan noting sales of interactive kiosks to have totaled USD 492 million globally in 2004, up 6 per cent on 2003 totals. Kiosk sales are predicted to see an 8.1 per cent CAGR from 2003 to 2010, with IDC estimating IT spending in vertical industries by corporations worldwide to grow from USD 965 billion in 2004, to USD 1.24 trillion by 2008. This 6 per cent CAGR growth will be driven by retail systems including self-checkout and photo kiosks, which accounted for 25 per cent of global kiosk sales in 2004."
Technology advances, improved infrastructure (the availability of broadband, especially), and public acceptance are cited as the key reasons for the proliferation of self-service terminals.