Jonathan,
I only know about the UK and the best advice I got was this:
The following will prevent roaming charges when accessing Wi Fi UK:
• If applicable, remove the International Long Distance and International Roaming features from your plan. This requires a call to your mobile service provider (probably AT&T).
• Have the phone in airplane mode
• Disable the 3G and Cellular Data
• Data roaming should be turned off
• Enable WiFi
• Turn off any automatic pushing. This prevents emails from being pushed to your phone.
• Turn off any automatic app updates
I don't use Skype, but many like using that service for making phone calls via the net (you'll want to check the details on their site).
I just purchased a phone card and used land-lines for phone calls that were not included in the hotel services.
Note - I made sure that I was connecting to the hotel or cafe wi-fi hotspot that was either free or cheap and did all my work in the Safari browser.
As far as any voicemails, text/images, etc.; I believe anything outside the browser connected in wi-fi above would accrue International roaming charges.
Connecting to MobileMe via the Safari browser in wi-fi allows iDisk access (I don't use DropBox, so not sure on that one).
Basic advice is therefore; (1) stay within the browser with iPhone settings set as listed above for low cost option, or (2) get a temporary package from AT&T to allow you to predict your costs. 😉