The race is about 51 Obama to 45 McCain for ballots actually voted, and 55 Obama to 44 McCain when requested ballots are included.
The diarist uses the partisan affiliations of those who have voted or requested ballots, and recent polling to determine the voting breakdown of each group. The imponderables in the numbers are the percentage of requested ballots that won't be returned (particularly in heavily Democratic Denver, which is getting many of its ballots out late due to technical difficulties), and the degree to which active registered voters (and inactive registered voters) will turn out on election day.
Absolute estimated numbers:
Obama early voting: 46,367
McCain early voting: 35,650
Obama received mail-in ballots: 243,840
McCain received mail-in ballots: 221,645
Obama requested mail-in ballots: 880,139
McCain requested mail-in ballots: 704,860
Total number of active registered voters: 2,602,815
Typically, one would expect 80%-90% turnout for active registered voters in a Presidential election year, and considerably less than 100% return rates for requested ballots.