I'm running a training course next week and will be basing it on the nightly build of QGIS, so I wanted to quickly build an installer for use on the course. I simply ran these commands (starting in a checkout of the Quantum GIS source code).
sudo apt-get install nsis cd ms-windows/osgeo4w ./creatensis.pl qgis-dev cd ..
Now you should see the new installer listed:
1.1K 2012-05-03 13:56 python_plugins.nsh 25K 2012-02-01 09:01 QGIS-Installer.nsi 111M 2012-05-03 14:13 QGIS-OSGeo4W-1.8.0-14615-Setup.exe <-- new installer 9.4K 2011-06-05 00:54 QGIS-Packager.bat 744 2012-02-01 09:01 quickpackage.sh 11K 2011-06-05 00:54 README.html 7.1K 2011-06-05 00:54 ui.nsh
Be aware that the process takes some time and doesn't really show much on the screen other than an dump of the nsis file created while it is happening. Just sit tight and wait for the command line prompt to return to you.