axis.line only changes one axis?

[statzhero]

After changing the default theme_grey to

axis.line = theme_segment(colour = "grey25")

only the x-axis has a grey line.

E.g.
qplot(carat, price, data=diamonds)

[wch]

I think this is the same underlying issue as #520, and should be fixed with the gtable rewrite in the next release.