Thank you for the writing.
RadRibbonBar does not support such functionality, because RibbonBar's layout is performed in the ExpandableStackLayout class that always align items to the left.
You could try to work around this by setting Margin for this group. for example:
.Size.Width / 2 , 2, 2, 2);
Please refer to the attached sample project.
Q2 2009 is an old version and we do not support it and recommend that your try our latest release Q1 2011, which we released earlier this week.
the Telerik team