Both of the the rovers are near to the Equator, while the seasonal change has more of an effect on Spirit than Opportunity (Spirit is about 14.5 degrees south, Opportunity is 1.9 south). Winter solstice for the southern hemisphere was around Sept 22, so days are getting longer now.

And aphelion has passed as well (that occured back in early August), so Mars is now getting closer to the sun. Perihelion isn't until next July, but its good to know that the situitation is improving. And, since dust storms really start to kick up around/after perihelion, hopefully the rovers won't have anything to worry about in that regard (until then, dare i hope they would last *that* long?).