The wording seems find to me.
I'm not so sure that is will be as noticable as we would like - it will scroll
off the top of the screen after a large output. And in cases like reported
here:
https://lists.gnu.org/archive/html/pspp-users/2016-02/msg00007.html
The user would not have seen the message until after experiencing the bug, if
at all.
How about we implement Ben's suggestion AND have an abridged version of his
words
on the title-bar?
The abridged version could read: "This is a development version. It may contain
bugs."