I believe that missing vet was 28, which was the last of the sub-100 teams to register.
Matter of fact, let's run the counts and see how things are faring by vintage:
Shuffle Era (1-204) 1 missing of 110 active teams in 2007 (0.91%)
1999 rookies (207-335) 0 missing of 63 (0%)
2000 rookies (337-488) 3 missing of 74 (4.05%)
2001 rookies (492-716) 10 missing of 97 (10.31%)
2002 rookies (743-999) 9 missing of 105 (8.57%)
2003 rookies (1000-1237) 16 missing of 98 (16.33%)
2004 rookies (1241-1495) 24 missing of 135 (17.78%)
2005 rookies (1501-1708) 27 missing of 155 (17.42%)
2006 rookies (1710-1980) 48 missing of 205 (23.41%)
2007 rookies (1981-2287) 54 missing of 255 (21.18%)
Not all the numbers line up perfectly; FIRST removes from the database I can see teams who haven't competed in three years (like
267, which last competed in 2004), so the actual cutoff might be a little different. Similarly, I might've miscounted the number of active teams of a given era; feel free to check my counts through whatever black magic you like.
By the way, the magic number is now below 200--it's
192, making for a 85.24% retention rate at the moment.