I downloaded and installed Quantum GIS 1.5.0 with GRASS using the OSGeo4W installer on my Windows XP machine. Unfortunately, when I go to Plugins>Manage Plugins I cannot find the GRASS plugin. When I browse to OSGeo4W\apps\qgis\plugins I can see grassplugin.dll, grassrasterprovider.dll and grassprovider.dll. So, I would appreciate it if someone could tell me how to make QGIS recognize the GRASS install.
I just installed QGIS 1.5.0 two days ago. The GRASS plugin shows up in my Plugin Manager and under the Plugin Menu. I used the Standalone Installer though, rather than the OSGeo4W Installer. You might want to try that route. (http://www.qgis.org/wiki/Download)
You also have to set up a GRASS Database through the "Plugins > Grass > New Mapset" menu in order to get the other GRASS Plugin Menu items to become active.
See also the draft "QGIS GRASS Cookbook" which is available here: http://grass.osgeo.org/wiki/QGIS_GRASS_Cookbook
In the long run it should go into the QGIS user manual as well.