The company for whom I work manufactures both massive electrode and iridium fine wire sparks plugs for reciprocating aircraft engines. Rockmeupto125 is correct in his 2nd post. Iridium is a great material for long life as it sacrifices less material to ionize the gap between the outer and center electrode hence the "long life" ads you may have seen from various manufacturers. It also resists lead attack better than almost any other precious metal (much better than platinum) which is not as big of a deal in auto/mc engines but is a BIG deal in aircraft engines that use 100LL (low-lead, which is actually a heavily leaded gas).
The outer electrode is solid iridium, not plated as iridium is a poor electrolytic (or electroless) plating material. The outer tip of the center electrode is typically a small piece of iridium welded onto a copper center electrode. Gapping iridium plugs is totally do-able. What we, as a manufacturer, worry about is some idjit who applies too much force (ever hear of a "calibrated elbow"?) when gapping the iridium plug that he/she breaks the weld between the outer electrode and shell.
If you're curious, we created a website with a decent amount of info on our aviation spark plugs at www.autoliteannie.com. Might answer a lot of your questions about spark plugs. Not trying to sell anything here, just providing some more insight into spark plugs.
Don't hate me because I'm in sales...