Feeling truly pedantic, I went and found the official ruling. And it's bugged!
U.3.Ruling.1 - You cannot use a coin with two heads (or tails) or a die which does not have 6 different values, as per Rule U.3.3. D'Angelo 1998/10/23
Which does nothing to prevent you from having a die marked 6,7,8,9,A,toothycat. --Vitenka Or, as has actually happened; Arrow, Blank, Cross, Explosion, Skull, SquigglyThing?. (We rerolled.)
In this case, nothing to stop you doing so, but not much to benefit. However, with something like MTG: Spark Fiend, using dice with the values 10,20,30,40,50,60 would be quite an advantage, and there's nothing in the Unglued rulings covering those cases. But sooner or later, you have to appeal to some degree of common sense on the part of the players... --AlexChurchill
The only really sad thing about this is that StuartFraser knew this ruling without having to look it up, and was wondering how long it would take before somebody quoted it.
This is one of a set of HomeMadeMagicCards. There is discussion on that page of the others :)