Q:

In SQL / MySQL, are there reasons not to put one-to-one relationship in the same table?

One-to-one relationship could usually be stored in the same table.  Are there reasons not to store them in the same table?

A:

Number and type of columns. There is a limit on the size of the columns in a table. See here. There is a maximum of 8,060 bytes per row.
Very large tables can also affect performance and can be difficult to optimize and index well.
This is apart from keeping data the is conceptually different, apart from each other. For example, a country and currency have a 1 to 1 relationship (illustrative example, I know this is not always the case). I would still not keep them together.

A:

You'll find some information about when it's useful to create one-to-one relations under http://onlamp.com/pub/a/onlamp/2001/03/20/aboutSQL.html
The most important thing is following:

The key indicator of a possible need
  for a one-to-one relationship is a
  table that contains fields that are
  only used for a certain subset of the
  records in that table.

