just fished around a bit on sdn for more info on the Yale waitlist from last year and it seems to jive with what you're talking about. Last year, they eventually got around to tiering it off (1-3) with tier 1 virtually being in, tier 2 being the maybe, tier 3 being the lower chance. There also seems to be a "high waitlist" that signifies the top of tier 1.
the bad news is that there has been great year to year variability with last year seeming like it got to the end of tier 2, the year before barely getting into tier 2, and 3 years back going well into tier 3 with 40+ people off the waitlist. the reason is most likely due to the strick 100 figure that yale needs to stick to for its class size and so it gives out initial acceptances relatively cautiously (not as many initial overcapacity acceptances) but then fills in with an active waitlist. the good news is that it seems mr. silverman is hands on about this and makes sure to talk to each waitlistee that calls (eventually).
this all seems to have started last year in early may with them saying it was behind schedule. here's the link to the long discussion:
http://forums.studentdoctor.net/showthread.php?t=113874&highlight=yale+waitlist
good luck guys