Lamby: They're all pronounced the same, so I'd go with the accent theory. :)

rain, rein, reign
to, too, two, 2
for, four, fore, 4
lead and lead are pronounced differently!
lead, led
feat, feet, fete
read and read are pronounced differently!
read, red
read, reed

I'm surprised you guys didn't have the peeve with "our" and "are" as I see them mixed up all the time.
And why is it in English some words have acceptable alternate spellings and others only have one accepted spelling?
Honour or honor are OK, but don't you dare spell congratulations with a "D"