Not sure if this is related to the twenty twelve theme or the core of 3.5. Thought I will post this here - just in case if it can be fixed in time for the release version. Also it seems it might be connected to this issue that is addressed in Trac here
Thumbnail pictures are looking skewed on Internet Explorer 8 (see screen dump.
They look fine on IE9+ and other Browsers.
For example the thumbnail on the right hand picture 387 x 86. It is generated by the tag:
<img width="1024" height="227" src="http://paddler.co.nz/wp-content/Imagefiles/2012/12/Kawhia-Harbour-010-1024x227.jpg" class="attachment-thumbnail" alt="Radio check">
It seems to happen because IE gets confused about which height to calculate (it takes the original height from the 1024x227 dimensions instead of the scaled down height that comes with the actual visual width of 387px).
If you want, you can check the live site to see what I mean.