I have my N900 now for 2 weeks and, after the usual stumbling, I managed to solve the playlist issue the EASY WAY:

-1: First install the excellent CL (command line) player called 'mplayer':
This is without a doubt the most universal mediaplayer available to maemo.

From now on, no matter how you proceed with the playlist issue, you'll always be able to enjoy the benefits of 2 players that can operate simultaneously: I do that: I watch video using mplayer (it plays all kind of formats the standard player is incapable of) while listening to additional music using the standard mediaplayer;

-2: Install the graphical shell around mplayer known as 'KMplayer', as not all of us are CL-wizards: One likes to operate from the desktop;

-3: All us musiclovers lived in a stone-age once, where we walked around with mp3-players that could only follow directory- and file-order: Hence we organized our affairs in directory-structures that made sense to us. This organisation we migrated from machine to machine, and now it ended up in the N900, maybe on an SD-card that was in the previous hardware. So, assuming all that:

a) Open KMplayer;
b) Tap the menu bar on top called 'kmplayer';
c) From the kmplayer-menu choose 'Generate Playlist';
d) From the Generate Playlist-menu choose 'Directory Media Scanner';
e) A directory-browser is presented to you: Browse 'till you find;
f) To open any directory INCLUDING all of it's subdirectories for playing, tap it when it is 'buttonized', right next to the 'back'-arrow ...

... and there you are: Your stuff is played to you just as you organized it ...

Henceforth you can take another advantage, but I'll describe that in a next comment, as this is my first one, and I don't know yet if length is limited here.