Actually there is such a method -- and I've shown it here -- http://spacefold.com/lisa/post/2007/10/03/Changing-the-Sheet-names-in-SQL-Server-RS-Excel-QnD-XSLT.aspx -- and there are lots of followup posts on my blog because people seem very interested in this technique.
But you may not like it.
Basically the idea is to use XML versus Excel as your output format and use XSLT to transform the XML -> SSML (Excel's 2003 XML dialect). It's quite easy, and you can save the XSLT information with your report definition so it is applied whenever XML is exported, it's not a separate option the user has to pick. The whole configuration and dev process is spelled out as a walkthrough in that post.
Of course, in later posts, I've let people know that in R2 this is no longer necessary *IF* the only thing you care about changing in Excel is the sheet names -- but it's still a great technique to know, because there's *lots* more you might want to alter in the default Excel presentation that SSRS provides.