In reply to im off:
That should have vertical drop-outs and I've never seen them worn - even taking wheels in and out as much as I have on my bikes over the years.
Are you sure it isn't the rear wheel that's either slightly buckled, or incorrectly dished?
If me, I would take the cassette off and put the wheel in back to front and see if it off-sets the other way - take it to a decent LBS/wheel builder to be checked on a proper jig if not sure.
Also, are you sure the tyre is correctly seated? Might be an issue if the tyre is very tight on the rim - some rim/tyre combos are notorious for fit and this could be a symptom.