Okay, so I was bored this afternoon and decided to try and figure out exactly how this Vader was the "500th Figure." While I still don't have an exact answer, I have gotten very close to the number 500 by taking several things into account.
Things to take into account (at least the way I came up with them) are that I did not count repaints (ala the Animated Clones or the Saga BDs), rereleases (Imperial Officers, RFTs, Koth, and Teebo in Saga or most of the OTC), or exclusive figures (Star Tours) in this tally. Also, I only went with the single carded figures in the basic line to arrive at this.
Okay, to start with, I went from the only other figure we know of that had an actual number attached to him (arbitarily though it may have been): Boba Fett from 2001. On the back of that figure's box is a list of the first 300 figures, and going by that, figure number 299 was Plo Koon. Figure 301, by my estimation, would have been Aurra Sing (who was in the first wave of figures that followed Koon that winter of 2001).
From Sing (301) to the end of the POTJ line (the AOTC SP figures in 2002) gives us 334 figures total.
Saga 2002 gave us 53 figures total, for a running total of 387.
Saga 2003 gave us 34 figures total, for a running total of 421.
Clone Wars 2003 gave us 10 figures total, for a running total of 431.
Saga 2004 gave us 21 figures total, for a running total of 452.
Animated Clone Wars gave us 11 figures total, for a running total of 463.
Here's where it gets a bit tricky...the OTC:
Vintage OTC gave us 12 figures total, for a running total of 475. But it's the basic OTC where the trickiness comes in...
There were 7 original figures in the OTC line that were not reissues. That gives us 482 total. The problem is that there is a figure that was a basic figure (Holo Palpatine) but it was also an exclusive; and there was a figure that was previously a pack-in (the Scanning Trooper, which theoretically was never released on a card before and would count now since it was) released in the line. I ended up including both figures in the total for the OTC, bringing our running total to a height of 484.
OTC gave us 8 more unique figures in 2005 for a running total of 492. There were four ROTS Sneak Preview figures which would bring the grand total to 496 unique figures released on cards including Hasbro's previous 300 figures up to and including the 300th Fett and counting each unique figure sculpt released on a card since then.
So, just prior to April 2nd, 2005, by my estimation, there had been, not counting variants, paint scheme differences, head sculpt differences, reissues, etc, 496 action figures in the SW line.
That's still three short of Vader being 500. Anybody see something I'm missing, or is Hasbro just going to claim that Obi-Wan, Anakin, and Yoda from the start of ROTS are 497, 498, and 499?