Good question...I'll be able to report on that this Friday, since I went ahead and ordered one for my Mother (she was originally going to get the previous ad-supported Kindle @ $114).
The ads, as stated on Amazon's website, are only shown on the screensaver screen and as a small block on the bottom of the home screen. No advertising will be displayed while reading. But when you look at the pricing of the non-ad version, it's only $25 more expensive. That's the same price for the 2 year service plan.
The Adds are not bad at all and the adds are good. Ive goten free books a buy one get one movie rentings. I like the add and they are just on the screen saver you never see them when it is on and reading.
also if you fall asleep like me well reading and prone to dropping the book off you bed you may want to spend that $25 on the 2 year service but I have dropped me 10 times and it still works great.
I'm waiting for the Kindle Touch. I just have to try to not go into a Best Buy and see what the new Kindle is like until then, otherwise I'll probably walk out with one.
Just finished setting one up for a Christmas gift. $80 got me the Kindle and 30 free "tween" aged books. The device is better tham my Kindle 2 and way light in weight. You can't go wrong with one of these.