I've been using the method of adding next actions that I have on my immediate radar to an additional category called "!Today". These next actions have 2 categories on them; the context they apply to, like @ Computer as well as the !Today category. The use of the exclamation mark, makes these items appear at the beginning of Outlook 2003's task list.
This method has been described in these forums a few times. I'm not sure who started it, so please accept my apologies for not giving due credit.
I tried to get the named flags operating as well, but I could only get the different coloured flags to operate against e-mail, not tasks. When I searched MS Outlook's help I found an MS Office Online link that took me to a page that confirmed this:
Maybe I missed something here, so If anyone has further ideas, please let me know!