While Croatia is closer to EU membership than Serbia, my understanding is that formal accession to the EU is unlikely before 2011. Further, Croatia is not about to join the Schengen area imminently. My understanding also is that Sarmellek / Balaton airport in southern Hungary was not a great performer for Ryanair.
eu01 - I hope you'll forgive my ignorance, but what is the benefit for Ryanair of flying from Osijek instead of somewhere close to Belgrade or Sarajevo ? Are Osijek airport simply paying a large amount of 'marketing support' ?