It's easy to predict slower than normal growth for a new version of the browser. Microsoft prides Internet Explorer as an “industry standard” browser to begin with, so many businesses and users might be leery to switch until necessary, opting to stick with an older version instead. Moreover, Microsoft hasn't advertised IE8 much. They do have the advantage of being able to push it through Windows Updates if they want, but have hosted no public events to give any publicity to their new browser.
All in all, these things might slow growth for the browser but shouldn’t actually cause it to fall backwards. So why would IE8 lose any share at all? Is Microsoft facing adoption issues? There are reports suggesting that this could be related to compatibility concerns, as some early adopters are finding that IE8 will not display some web pages accurately. It's too soon to say for sure, but it's worth paying attention to.