just back from 8 days in Europe, Germany and the UK.
In Germany, I used Nimbuzz/Nimbuzzout (and Sipgate) for VOIP on Wifi. Very good quality, only slight lag. Sipdroid was no good, too quiet and poor quality.
Truphone has just announced their own VOIP is now compatible with the Galaxy S.
for the UK, we unlocked our 2 Vibrants before leaving, and I used UK Orange sim cards on their Camel plan. Bought 250MB of data and 100 mins local UK calls for 5 pounds each (lasts a month, though main account balances don't seem to expire, maybe need to top up once a year at least.) Camel lets you call abroad from the UK cheaply, around 6p/min back to the US.
Phone worked fine everywhere, but several things:
1. when you switch sim card, the gmail password needs to be re-entered
2. got 3G in Germany, but UK only worked on low 'g' speeds (GPRS?) - not sure if this is due to phone or orange, and another sim card might have matched the phone's frequencies. Was on wifi most of time, so not a big deal.
3. the calendar gets messed up by the time difference - in the UK, showed everything as 5 hours later, but now back in US, events are shown at the right time. This is a bug that needs fixing.
happy travels!