When you get your N900 just enable extras-testing or extras-devel and install it through there (remember to disable those repos after installing mypaint and the usual disclaimer goes for these repos - do it at your own risk, be ready to reflash, no biggy though)

Or you could download rootsh, when you get it, through extras repo and then when installed, open xterm and do this code:

sudo apt-get install mypaint 'enter'

(don't write enter just press enter after mypaint)...

And that should do it

Ohh and by the way it's 0.8.x that is out now and gets updated through repos if enabled testing/devel