Someone over at the Mixxx forums doubted the N900's ability to handle such an app, but i honestly think it can.

I can run traktor with 4 decks with no problems on my 1ghz laptop with only 512mb memory.

Putting your phone into offline mode might be a good idea to gain some more resources and help stop any unwanted communications interrupting the audio.

Oh and if the phone rings while your mixing, dont answer it, adjust your bpms and mix the ringtone into your set!

Years ago when i worked in a supermarket, the store phone would ring over the pa and sometimes would be in sync with the music playing. Classic.