Blair Bigger of Microsoft MCS Federal informed me today that a BIOS update gives Latitude D630 users access to 8GB of memory. This does of course assume 8GB is installed and you are running a 64bit OS.
Blair indicated it is the BIOS update dated 8/13/2008. Looks like it is sitting at http://support.dell.com/support/downloads/download.aspx?c=us&l=en&s=gen&releaseid=R194000&SystemID=LATITUDE%20D630&servicetag=&os=WV64&osl=en&deviceid=13457&devlib=0&typecnt=0&vercnt=12&catid=-1&impid=-1&formatcnt=1&libid=1&fileid=267710 on the Dell support website.
Go forth and prosper!