I recently came across these charts showing different browsers ability to make users adopt the new versions. As you can see this works very well for the Google Chrome browser having automatic updates:
FireFox is getting better at this also, though it seems many users stick with the 3.6 version:
However, Internet Explorer is really lost here, we see also no user adoption of the newer browser versions. COME ON, MICROSOFT!!
Sources: arstechnica.com / netmarketshare.com