A) I've heard repeatedly that nobody has been charged with a violation of the ban alone. I've heard of cases of people having a charge tacked on to other charges such as theft of guns, trafficking, etc.
B) The Ban runs out on September 14, 2004 unless it is renewed. That will require a majority of both the House and Senate as well as the President's signature. Considering how Bush is doing as well as redistricting I doubt it'll make it past the house. This comes up only 3 weeks before Bush comes up for re-election. So, if it lapses in 2004 then there will be no law in place for any future Democrats.
C) If it does sunset on September 14, your Rifle will gain amnesty should any new law take its place after that date. For example if Bush is cheated in 2004 then you'd have 4 Months and 6 Days to Build and document Pre-Ban Status before a Democrat takes office and changes things.
D) It is my personal opinion that placing the burden on the defendant is unconstitutional, but as numerous people have stated that is how it is. As for Status, showing that the lower was around long before the ban will likely convince most people unless the ATF can show it wasn't assembled. A lower made on Sept. 12, 1994 or within several months of that would likely be questioned as it would likely haven't been assembled.
E) If it came in a kit with a receiver or the person bought a kit and receiver before Sept. 14, 1994 then it is Pre-Ban.
F) Common sense will show that if it was produced long before the ban then there'd be no reason it at the least wouldn't have been with all the extra parts. For example, why would I buy an AR lower unless I had an upper to put on it. Most if not all Uppers made before 1994 were Pre-Ban configurations (Threaded Barrels and/or Bayonet Lug)