The rate I was quoted by AT&T for international data roaming in the UK was $0.0195/kb. So given my usage, without the plan I'm looking at a $400 for the 8 days I'm gone whereas the plan lowers it to $24.99.
Furthermore, the customer service rep said both the global voice and data packages are pro-rated by the day. So, if I activate them in the airport on Saturday and then cancel them on the Sunday I return I'll only pay about $6.66 (($24.99 / 30 days)* 8 days of usage) for the data and $1.59 for the voice.
2 points, one it's always cheaper to go with ATT's int'l data plans. However, I don't think it is prorated in a linear fashion. IF you use 10 MB of your 20 MB plan in whatever 1/4 of a month is, you will pay more than 1/4 of $24.99 (probably half the $24.99 rate since you used half of the 20MB.
Also, be careful about starting service at the end of your billing period. For example, if you were to start your plan 2 days before your billing period ends and then continue your trip 6 days after the end of your billing period, you will be charged for 2 months service on a prorated basis. I got burned on this one and now make sure I start my int'l data plans on the first day of my new billing period even if it means I'm 2 or 3 days into my trip.
Also, it's a good idea to cancel the plan as SOON as you hit US soil or it will continue to "tick". While I'm all for tethering, it certainly is NOT the answer for surfing the net on your computer while on an int'l trip. You will VERY quickly go over your plan limit. Also, it's VERY important to turn off roaming when you're not using it on one of these plans. Otherwise, you're going to "meter" extra data that will zoom up your total.
The other point is go install Skype if you haven't done so. All you have to do is find a free wifi signal floating around somewhere and connect to it. Then, you can make int'l calls back to the states or anywhere in the world for around 2 cents a minute. About a year ago I made several calls back from Greece with this method.