WPF's DataGrid uses a TextBlock for the cell when the row is not in edit mode. When it is in edit mode, a TextBox is used. Neither control has a property that affects the vertical alignment within the control, but using a style you could set the VerticalAlignment of the TextBox itself to be center-aligned. This might cause undesired side effects though since there will now be areas of the edited cell not covered by the TextBox.
<DataGridTextColumn ...>
<DataGridTextColumn.EditingElementStyle>
<Style TargetType="{x:Type TextBox}">
<Setter Property="VerticalAlignment" Value="Center" />
</Style>
</DataGridTextColumn.EditingElementStyle>
</DataGridTextColumn>