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Since * is a greedy match (i.e. zero or more times) it would probably trigger most if not all the time. So it looks like some friendly fellow has set up a sitewide redirect to yahoo.com.
Does your site work with it in?
Is your site hosted by yahoo.com? Perhaps it is part of a bigger solution from them.
You can always comment out the rule rather than remove it completely. Do this with a # at the start of the line.
#RewriteRule /*$ [yahoo.com...] [L,R]
This would disable that rule without removing it. If everything breaks you can remove the # and go back to how you are now.
[edited by: MattyUK at 6:37 pm (utc) on Nov. 28, 2006]