Haze, ironically, I just redid (win7 makeover, blah blah) one of these today, in fact it was an inspiron 1546 I think. Anyways, I can confirm the touchpad disabling feature.
Make sure you have the Touchpad driver installed, here's the link, just pick the Operating System you're working with first, then download it:
[link=http://support.dell.com/support/downloads/driverslist.aspx?c=ca&cs=cadhs1&l=en&s=dhs&ServiceTag=&SystemID=VOS_N_1500&os=WLH&osl=en&catid=&impid=]
Touchpad Driver[/link] (DELL Vostro 1500)
Once installed (might have to reboot), you can open the mouse settings, and you'll see a big DELL screen pop up with it's own mouse settings. You can set it so it automatically disables once a mouse is plugged in via USB, or you can enable/disable it at your own will.
Cheers, good luck!