Just turn off Roaming Data and it will use WiFi. I just got back from Europe, and can offer the following advice on AT&T's roaming plans. First, they are pretty good, considering. Only T-Mobile's is better (free, unlimited international data, but only GPRS speeds).
AT&T's roaming data plans are shown here: http://www.wireless.att.com/learn/international/roaming/affordable-world-package s.jsp#data
The somewhat tricky part is that they are pro-rated based on where in your billing cycle you enable them. For example, if you add the 120 MB plan half way through your billing cycle you will only get 60 MB over the next 2 weeks. So it is usually better to make the effective date of the plan the start of your billing cycle, even if you won't be leaving for a couple of weeks. If you add the feature online there is an option to do this, even if the day you add it is after the start of your billing cycle.
I added the $60, 300 MB plan for 12 days in Europe, and used 295 MB. If you go over it isn't that bad anymore; they just tack on another 150 MB for another $30. Considering the cost of a trip to Europe the data plan is small change.
One other bit of advice. Reset the usage counter at the start of your trip. Take screen shots of usage periodically throughout the trip, and one final one when you get back, showing the total International roaming usage. Mine showed 281 MB, vs the 295 MB that appeared on my bill. I called AT&T about the discrepancy, and they said that if I had gone over 300 MB on the billed amount but had a screen shot of my usage as reported on the phone was less they would have adjusted the bill.