Centering multi-line strip text in lattice

I'm having trouble centering multi-line strip text in lattice. As the
code below demonstrates bounding box of the text is centered within
the strip, but the first line isn't centered in relation to the longer
second line. The "adj" argument to par.strip.text doesn't seem to do
much. Suggestions?

a=data.frame(
	x=rep(1:10,2)
	,y=rep(1:10,2)
	,z=rep(c('First Line\nLonger Second Line (1)','First Line\nLonger
Second Line (2)'),each=10)
)

xyplot(
	y~x|z
	,data=a
	,par.strip.text = list(cex = .75, lineheight=1, lines = 2, adj=.5),
)

Mike Lawrence
Graduate Student
Department of Psychology
Dalhousie University

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~ Certainty is folly... I think. ~