mattglet - 3:15 pm on Mar 8, 2010 (gmt 0)

My concern is that I wouldn't expect new versions of a browser to break a site that was previously ok In the older versions.

One of the major changes in 10.5 was the rendering engine... didn't you see the changelog or view the announcements? And Opera sure isn't the first (or last) browser to have things render differently in different versions.