That page of Sebastian's hasn't been updated for some time. For the current status, please check the N900 kernel status page. The n900-modem driver was merged in 3.16. If I'm honest, I haven't actually tested the modem yet.
I've only really tried Enlightenment which has an option to optimise for mobile devices.
I'd like to draw your attention in particular to this issue before you go off and install ALSA or similar and blow up your speakers.