IsTabStop="False" is not working for the AutoCompleteBox.
I was not going to use this control because of this problem.

I found a solution by googling elsewhere and will post it here.
Instead of using IsTabStop="False" use: KeyboardNavigation.TabNavigation="None"

NOTE:
When RadComboBox has IsEditable="True" then, like AutoCompleteBox, IsTabStop="False" does not work.
This solution will also apply to the RadComboBox problem.

I have my working AutoCompleteBox set up as below.​ I have bolded the code that will make tabbing skip the control.

<telerik:RadAutoCompleteBox Grid.Column="1" Grid.Row="0" 
   SelectionMode="Single"         
   WatermarkContent="Type a bylaw"    
   TextSearchMode="StartsWith"          
   TextSearchPath="Code"
   AutoCompleteMode="SuggestAppend"
   ItemsSource="{Binding Bylaws}"
   DropDownItemTemplate="{StaticResource BylawSearchTemplate}"
   SelectedItem="{Binding SelectedBylaw, Mode=TwoWay}"
   DropDownWidth="Auto"
   Style="{StaticResource EditRadAutoCompleteBox}"
   KeyboardNavigation.TabNavigation="None" /> 

Hello Kellie,

Thank you for the information. We confirm this is a valid workaround for the issue with IsTabStop="false". I've updated your Telerik Points as a small sign of our appreciation for sharing the workaround with community.

Feel free to contact us in case you have any other problems or concerns.

Regards,
Rosen Vladimirov
Telerik