I am trying to find the best way to get from Ibiza to the Greek islands. We will be leaving 1st May 08 and only have 3 days to spare so we need to pick one island. Either Mykonos or Santorini, which ever is easier to get to.
So far I have not had much luck, with the trip needing at least 2 stop overs. Is there an easier way to get from
Ibiza - Mykonos
or
Ibiza - Santorini

I would have thought that you have absolutely no chance of finding a direct flight between Ibiza and Mykonos or Santorini. I've checked all the websites that I have bookmarked and can't find anything direct between Ibiza and anywhere in Greece. There would be very little demand for such a service. It seems a very costly exercise, both in terms of money and time, to go there for such a short stay.
There have never been direct flights from Ibiza to the greek islands. The normal way goes via Barcelona and Athens (www.clickair.es, www.vueling.com). During the summer season, it is sometimes possible to fly from Ibiza to London, Paris, Milan or Germany and from there directly to Mikonos or Santorini, but in most cases, the timetables of the too legs don't mach.

Two stopovers is going to be the minimum to Athens from Ibiza. Besides flying back to Barcelona or another Spanish international airport, if you can find a flight from Ibiza to another international airport in Europe that has connecting flights to Athens you might be able to make better time. Aegean Airlines has flights to Athens with connecting flights to both Santorini and Mykonos from Rome, Milan, Frankfurt and Munich, and you should be able to take advantage of Aegean's online discount fares. AlpiEagles has a flight to Athens from Venice. EasyJet has cheap flights from London to Athens. I suppose you might find an excursion charter flight from one of the larger European cities directly to Santorini, which would cut out one stopover, but I'm not sure if they will be operating charters in May.

My first thought is why anyone would want to go to a Greek Island for only three days unless already in Greece? As other posters have pointed out, it would be a costly and time consuming effort to get from Ibiza to Mykonos or Santorini.
Although I, myself, prefer the Greek Islands, I have also been to Ibiza, and enjoyed it very much. If you want to visit a second island, how about Formentera? It would be a good contrast, has stunning beaches, and best of all is only a short ferry ride from Ibiza Town.