You can change it without root by using a different launcher that allows custom home screens, like adw, but that changes everything. As for leaving the stock motoblur launcher, you cannot change it without root. Once rooted you have to hack the framework files to change it; if it has been done, you will find the hack over at XDA.
Like this one:
[THEME] Black Bar Bell. (Gingerbread) - xda-developers
Note I have not tested this and it can BRICK your phone and void your warranty if something goes wrong, plus you will not be able to update your phone when the update comes out unles you have a way to return back to stock.