I would bet early 07 at best. E90 coupes will not likely be here until late 05 or early 06 so the M3 will be behind by some time - it always is and its always behind schedule. JMHO but I'd wager on it.
On the depreciation thing look at what 98/99 M3's are going for today (these were $45K cars new - still getting well over 50% after 5 years). While the E46 will likely take a bit of a hit just like the E36 did when the E90 comes out I doubt it will be substantial especially since you can be sure the E90 will be more money and in very tight supply for the first couple of years of production anyway (assuming it lives up to its performance heritage and its not too badly bangled - later being a BIG assumption).
There are plenty of dealers with either cars on the lot or with open allocations - your dealer sounds like a real work of art.