I've been playing with wikipedia again when I should be doing other things. Today I was looking up my old university, St Andrews in Scotland. Tied up in the history of the town is St Regulus (aka St Rule), and I thought it would be fun to read the wikipedia entry on the saint, since JKR is known for giving her characters names which mean something. Reading the entry made me cry "Dude!" repeatedly. I think I may have found something worth pondering:
Saint Regulus was galvanized into action by a visionary dream that Emperor Constantine had decided to remove Saint Andrew's relics from Patras to Constantinople. Warned by an angel, he was to move as many bones as far away as he could to the 'ends of the earth' for safekeeping.
Now, if we're assuming Regulus Black is R.A.B. (which seems extremely likely), then the above can be considered very significant. R.A.B. found and moved at least one of Voldemort's horcruxes to an unknown location (alright, so it's probably the locket mentioned in OotP at Grimmauld Place, but anyway...). It has also been suggested that the term "hallows" implies a saint's relics or bones. So I'm thinking the final horcrux might be the bones of a Founder of Hogwarts, or maybe something that was buried with a Founder.
*dances madly about at own cleverness*