To reproduce:
Add RadAutoCompleteBox
Add AutoCompleteItems
Change the theme to Office2010BlueTheme, Office2010BlackTheme, Office2007SilverTheme or Office2010SilverTheme.
Write something - looks good
Autocomplete a word and now write something - text is a few pixels lower.

Workaround:

void radAutoCompleteBox2_TextBlockFormatting(object sender, TextBlockFormattingEventArgs e)
{
    TokenizedTextBlockElement token = e.TextBlock as TokenizedTextBlockElement;
    if (token != null)
    {
        token.Margin = new Padding(0, 1, 1, 1);             
    }
}