I've got alarm bells ringing now... matey's post above said basically never to use PVA on walls as the tiles stick to the PVA layer, and not the wall itself.
I've been tiling the bathroom, and I've PVA'd the walls in advance
But... I did it because the manufacturer (am I allowed to say their name? Well... it begins with U and ends in D) states on the back of the tub of "Waterproof Wall Tile Adhesive":-
"Prime porous surfaces with 1 part UniBond PVA diluted with 5 parts water, and allow to dry".
Well... I've just rung their "technical helpline", and the guy there said it was imperative that the walls are cleaned up so that any traces of old wallpaper paste etc are thoroughly removed. Then, the diluted PVA (5:1) can be painted on, and at that level of dilusion it will absorb into the wall. The idea being that it won't leave a film as such.
Anyone got any further comments on this. I don't know if I'm happy or not having PVA'd the walls already?!