header-line-format interpreting tab chars differently now?

[Mathias Dahl]

Until today I used a CVS Emacs from 2004-07-26. Today I
switched to a newer one and suddenly tab characters in
header-line-format is displayed differently.

Evaluate the following and you will see what I mean:

(setq header-line-format "Here is a tab:[\t].")

The result in the new version is that "^I" is displayed, in
blue text, in the buffer header line, while previously it 
was displayed as a whitespace.

Doing the above works differently in the two versions of
Emacs. The new one is from 2005-01-30. I obtained both
versions from the nqmacs-project on Sourceforge.

I don't know which way is "right" but something has happened
during the last six months and I wanted to bring up the
issue.

/Mathias