Grapes in yeast starter

... that the yeast on grapes and other fruits is not the same as the yeast that you want for sourdough. So although that will give you some activity at the start, (i.e bubbling and such) this yeast eventually dies off and the real yeast has to then take over anyway. So skip the whole grape thing and go right to the grain yeast.

See gaaarp's most excellent thread on starting a starter here:

Starting a Starter - Sourdough 101, a Tutorial

and pass over the messy week-long grape soup step completely since it really doesn't get you anywhere that you want to be.

And make some grape jam with the grapes you have, it'll be very tasty on the sourdough bread you'll be making in about a week or two.

The previous post might go a long way in explaining my experience.  I read about creating starters using grapes here:

http://www.exploratorium.edu/cooking/bread/recipe-grapeyeast.html

Here's a photo of it about a day before it got really rolling:

(note the speeding ticket summons are for the wife, not me :)

About a day after this pic was taken it turned into a frothy foaming beast.  I strained the grapes out and started feeding it like a normal sourdough starter.  This guy had enourmous power, but always smelled very much like alcohol and never had a good flavor.  I used it a few times, but gave up on it as a nice experiment but not really useful for making bread. 

I agree with Rainbowz's post above, recommend a regular sourdoug starter for making bread.  I maintain a white and a whole wheat starter, I use one of those two with great success depending on the application.