Earlier quoted context omitted.
Hopefully you don't have an auto-increment int (2^31) as a PK, or you'll be worrying sooner than you think.
That's one reason why switching auto-increments to bigint does help (a lot): http://dev.mysql.com/doc/refman/5.5/en/integer-types.html
That is interesting to know, I have been setting my index's to bigint for a long time.