No Scroll bar on Modal window in a Centered Layout
When modifying the layout of SharePoint 2010 to be centered, as opposed to the standard layout, there are many modifications that need to be performed. One such modification is to enable scroll bars on modal windows (windows that appear when adding/editing items in a list). By default, modal windows will not have a scroll bar in centered layouts. To resolve this, the master page will need to be modified by removing 'scroll="no"' from the BODY tag.
This is a screenshot of a modal window in a centered layout without the resolution
This is a screenshot of a modal window in a centered layout with the resolution
A good link to check for centered layout's is: http://erikswenson.blogspot.com/2010/08/sharepoint-2010-centered-fixed-width.html