With the release of Tiger, the window that reads "Starting Mac OS X..." (which appears just before the login window) has been changed to a "fake" window (see this hint). This window no longer shows what is actually happening, it just has generic text that is now possible to change, if you are willing to enable the root account (this can be done by opening NetInfo Manager, which is located in Utilities, and visiting the Security menu).
Once you've enabled the root account, browse to /System -> Library -> CoreServices -> Resources. There will be a number of different folders for different languages. Choose the one that your system uses when it boots up (probably English.lproj). If you have the Xcode Developer Tools installed, just open the file called SystemStarterUI.strings. Otherwise, it should be possible to edit it in TextEdit.
On the second line, it should say:
"SYSTEM_STARTING_MESSAGE" = "Starting Mac OS X...";
Where it says "Starting Mac OS X...," put in your own message (leave the quotation marks around your message). I wouldn't reccomend putting in anything strange like nothing at all, paragraphs of text, weird symbols, etc. (although I honestly haven't experimented with all the possible things you could put in there), but you can try.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050830114525532