I imagine the Top 25 at the end of the season is fairly static from year to year. I wonder if you took the top 25 from the past ten years, how many schools would have seven or more appearances...I bet close to half, at least.
Sounded like an interesting problem so I compiled the stats. These are for years 2002-2010 (AP only, sorry). Since there's only 9 years of data, I'm going to post teams with 6 or more appearances. The stats are only for years where the team made a top-25 appearance in the final rankings. If USCw went 0-12 in 2003 and didn't make the top-25, it won't count any of that in their ranking, points or wins/losses data.
Ohio State :
Average Wins : 11.0
Appearances : 9
Average Losses : 1.88888888889
Average Points : 1259.11111111
Average Rank : 6.11111111111
USC :
Average Wins : 11.375
Appearances : 8
Average Losses : 1.625
Average Points : 1370.75
Average Rank : 5.0
Oklahoma :
Average Wins : 11.25
Appearances : 8
Average Losses : 2.375
Average Points : 1163.25
Average Rank : 7.875
Texas :
Average Wins : 11.25
Appearances : 8
Average Losses : 1.75
Average Points : 1234.625
Average Rank : 6.625
Virginia Tech :
Average Wins : 10.375
Appearances : 8
Average Losses : 3.125
Average Points : 810.375
Average Rank : 13.0
TCU :
Average Wins : 11.2857142857
Appearances : 7
Average Losses : 1.42857142857
Average Points : 760.142857143
Average Rank : 13.7142857143
Boise State :
Average Wins : 12.4285714286
Appearances : 7
Average Losses : 0.714285714286
Average Points : 991.857142857
Average Rank : 10.2857142857
LSU :
Average Wins : 10.8571428571
Appearances : 7
Average Losses : 2.28571428571
Average Points : 1175.0
Average Rank : 7.57142857143
Georgia :
Average Wins : 10.5714285714
Appearances : 7
Average Losses : 2.57142857143
Average Points : 1096.14285714
Average Rank : 9.28571428571
Florida :
Average Wins : 10.8333333333
Appearances : 6
Average Losses : 2.5
Average Points : 1044.5
Average Rank : 9.0
Auburn :
Average Wins : 10.8333333333
Appearances : 6
Average Losses : 2.16666666667
Average Points : 1068.5
Average Rank : 9.16666666667
Wisconsin :
Average Wins : 10.1666666667
Appearances : 6
Average Losses : 2.66666666667
Average Points : 730.666666667
Average Rank : 14.3333333333
Florida State :
Average Wins : 9.16666666667
Appearances : 6
Average Losses : 4.0
Average Points : 488.333333333
Average Rank : 18.0
West Virginia :
Average Wins : 10.0
Appearances : 6
Average Losses : 2.83333333333
Average Points : 688.666666667
Average Rank : 15.6666666667
14 teams. So yes, that's pretty static. In addition to that, there are 7 teams that make 5 appearances each. Of the 67 teams that have been ranked in the top-25 final rankings since 2002, the 14 above occupy 44% of all spots, and the top 21 schools occupy roughly 60% of all spots.
Occupy Top 25?