I don't know if N900 is enough, but when you would do that in scratchbox:

- You enter directory of your app sources (so when you would type "ls" you could see debian folder and files from you package or sources) So if path to debian/control file is /some/path/yourapp-1.0/debian/control, you should cd to /some/path/yourapp-1.0
- You type "dpkg-buildpackage -rfakeroot" and let it finish
- In directory one level above ( /some/path in this example case) you should see files yourapp_1.0_armel.deb, yourapp_1.0.changes, yourapp_1.0.dsc and yourapp_1.0.tar.gz. It works this way in scratchbox and you then need to just upload these files via extras-assistant web interface