alter table (mysql) adding column with default value of space (' ')
Either the rules have changed, or somebody missed something in compiler builds.

According to the MySQL Documentation on ENUM


  For example, a column specified as ENUM('one', 'two', 'three') can have any of the values shown here. The index of each value is also shown. 
  
  Value   Index
  
  NULL    NULL
  
  ''      0
  
  'one'   1
  
  'two'   2
  
  'three' 3  
  
  An enumeration can have a maximum of 65,535 elements. 
  
  Trailing spaces are automatically deleted from ENUM member values in
  the table definition when a table is created.


You are using MySQL 5.1.49. That build probably errors out on seeing a one-character string with a blank.

MySQL 5.5 probably removes trailing spaces leaving an empty string.