It's still entirely possible that the 2008 presidential election could be decided as early as June of next year, if California voters decide to change the way their state allocates its electoral votes. Surprisingly enough, early polls still look decent for the ballot measure, which would divide up electoral votes based on how congressional districts vote. Hendrik Hertzberg explained the scheme here.
I'm not sure the change would really force the presidential candidates to visit California more often--most of the state's districts are so heavily gerrymandered that the outcome would still be all-but-determined in advance--but the measure would hand about 20 "free" electoral votes to the GOP nominee. Unless, of course, a major red state like Texas decides to make a similar change. But how likely is that?
P.S.: California Dems, for their part, are pushing for an alternate measure that would have the state award its electoral votes to the nation's popular-vote winner--taking effect only when states representing a majority of the electoral vote agree to a similar measure.