sounds as if most of your call will be WITHIN country. and so many countries. ideally one for every country, but that is going to be too expensive. cannot possibly buy a sim for every country. makes it hard. does not sound as if you EXPECT many inbound calls. not clear from your note whether you will be spending more time in any one or two countries. voda may be an option. calls will probably be short so the voda option may not be as valuable as it seems.
i notice in germany there are several new "roaming" sims on the market (sunsim, che-mobil, tourist mobil, etc.) inbound is 9 cents on many and outbound 39 cents per min + 19 connection. beats united mobile. don't know how that compares to geosim.
http://www.teltarif.de/arch/2007/kw13/s25438.html (in German)
best of luck.
stan