Well, effectively, France has called for Schengen to be suspended, at least temprarily, or that's what I've understood from the Sarkozy camp. (I may have missed a chunk - birthday weekend at York races with daughters.) I heard this evening on R4 that Germany is also making noises about re-instating borders. This is not before time I.M.O..
If we get our border put back to Dover etc. - then so be it, that's fine. It will take a bit longer to get off the ferries or trains, that's all. Then we need to nerve to ship 'illegals' back to where they came from.
I think that immigration has made people think seriously (possibly for the first time) about what being in the EU means, so in that sense it may very well be the defining issue.