Well in almost every case the answer is, when speaking to "ATC" on first contact you would notify them of the experimental nature of the airplane you are flying, per your "Operation Limitations."  These are the supplemental regulations you Must operate your Experimental aircraft to.  As far as I know, all new and most older 'ops Limitations' have this item.  Now when you speak on the radio at uncontroled airports the proper responce or innitial contact statement would be Blue and White Lancair bla bla bla, per the AIM.  Nobody cares what number is on the side of your airplane except the FAA/ATC.