My Constitutional Law is a little fuzzy (I took that course a LONG time ago) but I recollect that if the 8th Circuit upholds the constitutionality of the initiative (meaning, if they reverse Walker) then appeal to SCOTUS would be discretionary, via a petition for cert. Those are granted in only 2% or so of cases. But if the is found unconstitutional (meaning the 8th CCA upholds Walker) then the appeal to SCOTUS is a matter of right, and would therefore go to them for sure. Since it seems an 8th Circuit reversal is unlikely, this seems bound for SCOTUS.

Those with fresher, more detailed knowledge should chime in.