Outside Narvik, Norway (at Gratangseidet north of Bjerkvik, at the border between Nordland and Troms) is a road sign indicating the distance to Kirkenes along the E6 as 1042 km:
Kirkenes 1042 km
Outside Murmansk/Му́рманск, along the M18, there is a road sign indicating the distance to Saint Petersburg/Санкт-Петерб́ург as 1382 km (Google Streetview), and another one at 1434 km (presumably another route).
This forum post contains a photo of a sign indicating 2800 km to Perth.
I recall having seen a photograph of a road sign in Siberia, indicating ~5000 km-ish to Moscow, but I cannot find it now. However, this panoramio photo pictures 3177 km to Magadan / Магада́н.
What is the highest ordinary distance road sign in the world? By ordinary, I mean that special signs showing far-off cities don't count; I mean only road signs part of ordinary distance signage.
There is some discussion on a skyscrapercity forum thread and a aaroads forum thread.