What you're about to read is probably bunk.

Early vote in top 15 counties, 2004: 2,412,952
Early vote in all 254 counties, 2004: 3,779,124
Total early vote attributable to top 15 counties: 63.85%

Early vote, so far this year, in top 15 counties: 3,117,005
Early vote in all counties, so far this year, if the ratio from 2004 holds: 4,881,800

Total vote, 2004: 7,410,749
Early vote as a percentage of total vote, 2004: 50.99

Total vote, this year, if the early/late ratio from 2004 holds: 9,573,065

Difference, 2004 to 2008, if all that holds: 2,164,316 votes
Difference, 2004 to 2008, if if if: 29.18 percent

Source for real numbers: Texas Secretary of State
Source for extrapolations: Calculator, legal pad