The buffer was simply too small.
In 5.5 and trunk, the size is 311 + 31,
in 5.1 and below, the size is 331


client/sql_string.cc:
  Increase buffer size in String::set(double, ...)
include/m_string.h:
  Increase FLOATING_POINT_BUFFER
mysql-test/r/type_float.result:
  New test cases.
mysql-test/t/type_float.test:
  New test cases.
sql/sql_string.cc:
  Increase buffer size in String::set(double, ...)
sql/unireg.h:
  Move definition of FLOATING_POINT_BUFFER