Muahahahahah!
I'm not alone this week!
http://www.nissanforums.com/showthread.php?t=76303
Ummm, main thing to look for on your heater core is antifreeze spilling out of the foot vents. If you have it, IMMEDIATELY pull the dash covers off around the ECU and make sure the ECU isn't getting dripped on... wipe up any coolant there, then cover the thing in plastic until you have time to fix it. (saran wrap works great)..
I also highly suggest buying a heater hose splice at a parts store.. use it to bypass the heater core altogether for the time being to keep it from leaking any more inside your car. having no heat will also prompt you to fix it quicker.
Here's some more info from a buddy of mine over in the UK..
http://www.newshampark.org.uk/bignis...lownecufix.htm