class Profile < ActiveRecord::Base
  has_one :address
  accepts_nested_attributes_for :address
end

class Address < ActiveRecord::Base
  belongs_to :profile
end

>> p = Profile.new
=> #<Profile id: nil, name: nil, created_at: nil, updated_at: nil>
>> p.update_attributes(:name => "foo", :address_attributes => {:line1 => "my street"})
=> true

mysql> select * from profiles;
+----+------+---------------------+---------------------+
| id | name | created_at          | updated_at          |
+----+------+---------------------+---------------------+
|  2 | foo  | 2009-06-29 12:53:58 | 2009-06-29 12:53:58 |
+----+------+---------------------+---------------------+
1 row in set (0.00 sec)

mysql> select * from addresses;
+----+------------+-----------+---------------------+---------------------+
| id | profile_id | line1     | created_at          | updated_at          |
+----+------------+-----------+---------------------+---------------------+
|  3 |          2 | my street | 2009-06-29 12:53:58 | 2009-06-29 12:53:58 |
+----+------------+-----------+---------------------+---------------------+
1 row in set (0.00 sec)

>> p = Profile.first
=> #<Profile id: 2, name: "foo", created_at: "2009-06-29 12:53:58", updated_at: "2009-06-29 12:53:58">
>> p.update_attributes(:name => "josh", :address_attributes => {:line1 => "Other street"})
=> true

mysql> select * from profiles;
+----+------+---------------------+---------------------+
| id | name | created_at          | updated_at          |
+----+------+---------------------+---------------------+
|  2 | josh | 2009-06-29 12:53:58 | 2009-06-29 12:54:41 |
+----+------+---------------------+---------------------+
1 row in set (0.00 sec)

mysql> select * from addresses;
+----+------------+--------------+---------------------+---------------------+
| id | profile_id | line1        | created_at          | updated_at          |
+----+------------+--------------+---------------------+---------------------+
|  3 |       NULL | my street    | 2009-06-29 12:53:58 | 2009-06-29 12:54:41 |
|  4 |          2 | Other street | 2009-06-29 12:54:42 | 2009-06-29 12:54:42 |
+----+------------+--------------+---------------------+---------------------+
2 rows in set (0.00 sec)