Well, after searching Bugs on Metalinks ... My bug is comparable to an Bug that was reported concerning Date-fields on 11.1.
The solution is to first add the field with only defining the default-value and afterwards modify this field to set it to not NULL.
alter table testtable add testfield number(1) default 1;
alter table testtable modify filetype not NULL;
So it work'S as aspected and you don't need an explicit Constraint.
mfg
