a little will probably come out the top of the fan and go around the emi shield -- most should go out though.  for me, whenever I blow, it looks like the dust actually comes out the same back area that I blow into...?

Direct the air stream at the back vent as this will effectively reversed the flow of air through the heat sink. The dust will then try to make it escape through the other vent.

You can use fanGUI to monitor your temperatures http://www.diefer.de/i8kfan/index.html

Thanks for your help! So should I blow it at the back vent and not the bottom one?

correct ...  though it might not hurt to do the bottom one after you do the back one