why would you be flying a VFR flight to Dulles airport anyway? If you are flying a training flight you are better off going to Manassas (KHEF).
If you file IFR you wont have any problems. Dont forget you cant squawk 1200 in the ADIZ NO MATTER WHAT!!! That will earn you an automatic 30 day suspension.
If you are VFR with a discrete code, you need to hear the words "Radar Contact", Procede on Course" or Transponder Observed" before entering the ADIZ. ATC will never say "you are cleared into the ADIZ" because to them it is not a clearance. Of course, there is nothing wrong with asking yourself "Verify i am cleared into the ADIZ, and ususally most of them will say "yes sir, you are radar contact". EVery now and then you'll get on of the controllers who will respond "Sir it is not a clearance, but your transponder is observed". This whole paragraph is VFR only.
And finally, be careful of the FREEZE or FRZ zone. That is the white 10 mile or 15 mile ring around DCA. It comes close to Dulles. It is your resonsibility to avoid it at all times (if you are VFR). IF you are IFR, then it is up to ATC if they want you to avoid it or not. Penalty for busting the FRZ ring is rough, usually start with a 30 day automatic suspension. Most GPS's depict the ADIZ rings, and the inner FRZ ring.
Flying in and out of the ADIZ the first time makes anyone a little nervous, but once you do it you'll realize how simple it is.
Good luck sarg.