I have tiled up to an oak threshold in a doorway.  The problem is, I can see that the grout will probably crack against it over time as the wood will move.  Is there any way around this?  If I seal the oak with the oil / wax finish before grouting, then this will stop it taking in the moisture from the wet grout I suppose, but I still reckon it will move over time.  Could I add a bit of PVA to the grout while mixing in order to give it some flexibility?  Any help would be much appreciated.

Thanks for the advice mate, got myself some GT1 in.