Adobe: 25 Million AIR App Installs, Actual Number Likely Higher
sitepoint.com
sitepoint.com
I think offline/desktop access (whichever way you get it done) is an important step for the adoption of rich Internet apps.
I think the better solution is making desktop apps that emulate Internet apps. It's faster for the end user and it takes less effort for the developer. I think MobileMe is a perfect example of that. Make a powerful Internet app - but when you're not USING the Internet, just make sure it syncs with whatever app you're USING for your stuff.