When setting the background position in one unit and on :hover on a different unit webkit doesn't transition the background-position change. See http://dabblet.com/gist/1900938 for a test case. The colour transitions properly but the background position jumps from one state to the next. When declaring both background position declarations in the same unit webkit does transition the background position properly.
It's not currently possible to transition between unit types (here you're transitioning between a percentage and a length). Although this limitation isn't explicit in the specification (http://www.w3.org/TR/css3-transitions/) it is implied. Note that the CSSWG is currently considering whether and how to handle transitions between unit types. For now I've marked this bug as an enhancement request.
Is there some news about this? I am experimenting the same issues. Every other browsers are able to transition this properly.