Me thinks you need 3 days to settle and get the money into your account....however, you are 'locked' into whatever price you sold at immediately.....
example : I think you can sell stock on the 31st and still use for tax purposes, as the revenue/loss is derived from a sale dated in 2008.....even if, after 3 day settlement period it could be Jan/2009 before your funds are cleared.....
You can sell SRSR up to the last minute on December 31st to declare a tax loss. It has nothing to do with clearing of funds. Be aware that you either have to wait till the 31st day to re-buy the same stock you took the loss on or it will become a wash sale. However, if you don't want to take the chance the stock will run before the 31st day you can buy and equal amount or more shares in the same stock before you sell. That way you get the benefit of the tax loss write off and still hold stock in that company.