[GadgetKen]

Hmmm...you might consider addressing the voice and data as 2 seperate issues.

For voice would recommend an international call back sim like the United Mobile solution you thought of with an unlocked GSM quad band phone. You call out, it hangs up, you get a call back connecting you to your party. Have used United Mobile in several Caribbean countries. United Mobile would be free incoming and outgoing would be 39 eurocents a minute or 49 US cents a minute from most European countries to the US (depending on whether you want a dollar or euro denominated sim card). Outgoing call costs can also be reduced using the Callback World callback solution you thought of but does add a layer of complexity to calling (you still get a bargain using direct dial compared to Verizon/Vodaphone rental rates you mentioned). If you make a brief call or SMS from your phone at least once every 9 months on United Mobile you can keep your card alive for trips. You can also set up an account on the United Mobile website for web refills or even autorefill so you never run out of airtime. Please note that callback sims do not work well in some handsets and there's also some links on this website that help sponsor it:

http://www.prepaidgsm.net/forum/showthread.php?t=914

For data maybe try a Mobal or other data capable sim card with a GPRS modem card in your PDA at $20/mb or you can switch sims in the GSM phone, and tether. Their voice rates are extremely high, but it might be useful in a pinch for data only since their sim is free with $10 shipping. Still think the wifi or internet cafe solutions may the best and cheapest solutions for occassional access.

Other mobile data solutions might be to forward your work email to a text to voice or text to SMS service that can be retrieved from your cellphone.

Or just leave an email autoreply on your office email that you are out of the country and will only be checking email occassionally...for any urgent communications please leave a voice message on your office voicemail that will be checked frequently or you can be dialed direct at the following international cell phone number if it is an emergency but please note rates to call this number may be high depending on your long distance carrier.