alright, here is what happens

goto: http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=67257de128ce5110VgnVCM1000004718190aRCRD

on the right hand column, you would see "processing time". your case is considered within processing time if your RD is later than that processing time. you can file inquiry for your case only if your case is 30 days outside of CIS normal processing date. r u follow me?

For instance.for EB2 @ TSC. the normal processing date is now April 30, 2007. Your case will be considered within normal processing date if your RD falls after April 30, 2007. You can file with USCIS a inquiry only if your RD is before March 31, 2008. R u with me so far?
Give me a shout if you r still puzzled.

Hope this helps. Any correction would be welcome.