I'm not really convinced that hitting a magic "number" in a brokerage account has any meaningful significance for making a determination when to retire.
I believe this was mentioned in prior posts, but all that really should matter is how much passive income can be generated. It is also important to be confident that the amount of passive income will keep pace with inflation. Additionally, a buffer should be built in for "black swan" events, such as market down-turns, dividend cuts, unexpected expenses, problems with renters, etc.
For my personal situation, I would be more than happy to live on 2000 dollars per month of after-tax passive income (in 2012 dollars). I will probably continue to work until I have enough investments and rental properties to generate passive income of 2500 or 3000 per month (again, CPI adjusted to what a dollar is worth today in 2012.)
Since I am still really young, I think it would be a worth-while trade-off to work an additional couple of years to permanently increase my standard of living by 25% or 50% for the rest of my life. After my portfolio of dividend stocks and real estate rentals reaches a critical mass, if I continue to work for a couple of years, the savings will be compounding with great effect :)